Absurdly Sharp Blade: Finn's second and third swords. The second sword can sink into wood like hot butter, while the third is a family heirloom crafted from demon blood, and can cut through diamond. His fourth sword, the Grass Blade, is sharp and precise enough to cut his face into something as small as a price tag. As well as being sharp enough to destroy fear itself (by slaying the Fear Feaster at the end of "Billy's Bucket List"
The whole "Bubblegum now a child" thing. They patched it up pretty quickly due to the concept not working for the writers.
Jake's puppies looked like they were going to be an important part of the plot, as there was build up to them, but because they were rainicorn-dogs, they aged faster then Jake, becoming older than him. Everything went back to normal at the end of the episode. A couple of them have made reappearances however.
After five seasons of build-up and foreshadowing, Finn finally loses his arm in the Season 6 premiere. But then he gets his arm back four episodes later.
Accidental Misnaming: Tree Trunks keeps referring to Lemongrab as "Lemon Carb". This is not malicious, but more an indication of her borderline senility.
Fionna, appropriately enough, considering that she's a gender-flipped version of Finn, a professional adventurer.
Princess Bubblegum also proves she's capable of handling business herself when Finn and Jake are abducted. One gets the distinct impression that she could handle most of the threats she throws Finn and Jake at herself if she wasn't so busy administrating the kingdom and doing mad candy science.
In "His Hero", Billy is this because he thinks using violence against evil is pointless. He encourages Finn and Jake to find ways of doing good other than fighting evil, but it's difficult for them. Very difficult.
Jake himself makes a go at this in "Crystals Have Power". Unfortunately, he chooses a very inopportune moment because Finn is kidnapped shortly after.
Subverted with the Ice King. He appears fat with his robes on, but in "What is Life?", he is revealed to be very skinny, but with an abnormally large chest. In "I Remember You", he's fattened up to typically normal proportions, but he only lifts his robe up to specifically talk about how he's been working out for his chest.
The original Lemongrab became this between "Too Old" and "Lemonhope", with a horrible twist in his Character Development to boot.
Aerith and Bob: Played with. Either you have a noble title (Ice King, Duke of Nuts, Princess Bubblegum, Marceline the Vampire Queen, and many more) or you're Billy, Jake, Bob, and so on.
Played straight with Lady Rainicorn (where "Lady" is ambiguously presented as a name and a title at different times) and her parents, Ethel and Bob.
The episode "Go With Me" revealed Princess Bubblegum's first name to be Bonnibel, thus presenting a strange one-woman case: she went from "Betty" in the old prerelease publication to the more whimsical "Bonnibel" in the series.
In "Jake the Dad", Jake's puppies are named Charlie, Viola, Jake Jr. (who happens to be a girl), Kim Kil Whan and T.V.
An Aesop: Sometimes subverted, e.g. in Freak City and Witch's Gerden, but often played completely straight, albeit in weird ways. For example, Finn is taught not to break promises by being punished by the Gumball Guardians.
Affectionate Nickname: Princess Bubblegum gets a ton of them: PB, Peebs, P-Bubs, Preebs, Prubs... the list goes on. In "Reign of Gunters", Finn calls her a different nickname every time he sees her.
Finn in particular is a big nicknamer. He tends to go by initials: FP, LSP, IK... it's rather touching when Flame Princess returns the favor and calls him FTH in "Vault of Bones". There are also characters with simple nicknames, like Marcy and Lady.
The main story canon is set in a rejuvenated world following a nuclear holocaust known only as the "Great Mushroom War", which apparently mutated most surviving organisms. Marceline's childhood is set just after the war, 996 years before the events of the show, with the burning cities of man in the background. In "Holly Jolly Secrets", it is revealed that Ice King lived through this.
Simon:[The visions] tell me the secrets of the ice and snow. That the power of the crown will save me with its frost. I don't yet know what this means.
In "Finn the Human", Finn wishes that the Lich never existed. This episode reveals that the Lich was created by the apocalyptic war, and his presence made things even more apocalyptic. In the alternate universe where he never existed, the Ice King/Simon Petrikov stops the Lich from causing the world as Finn knows it by sacrificing himself. However, this causes the crown to become angry and freeze the entire world for 400 years. The setting could still be considered post-apocalyptic.
In the epilogue of "Lemonhope" (implied to be around a thousand years later) we see that Ooo itself is in this state: the Candy Kingdom had become an advanced Skyscraper City with automobiles and aircraft before being abandoned, Finn and Jake's tree is now massive and reaches well into the clouds, and the Ice Kingdom is a regular mountain range.
For some, Magic Man's history with his girlfriend. He obviously cared for her dearly, and misses her a great deal. Only in those moments was he portrayed as a human, hurting character.
Donny the Grass Ogre.
I'll never forget you, Finn.
Lemongrab in "Too Young" when Finn and PB beat him up and make him cry, in "You Made Me" when he goes crazy from loneliness, in "Mystery Dungeon" when he almost dies, and in "All Your Fault" when he almost starves to death by bringing all of his food supply to life to make himself a family. This poor jerk really can't get a break, can he?
Pen's form spring account occasionally gives away future developments and semi-frequently fleshes out the backstory of various characters. The same goes for the production blog; for example, it's why we know that dress of Marceline's in "Henchman" is made of whywolf.
Thanks to the production blog, we know that the race of Fish People that appears in "Susan Strong" is known as "hyoomans".
Alliterative Family: Jake's brother is named Jermaine and his father is named Joshua. Plus, Pen has played with the idea of giving Jake a sister named Jessica. Jake's mother's name is Margaret, though.
"Finn the Human" introduces one called Farmworld (the name is All There in the Script) where the Lich never existed, and as a corollary, the Mushroom War that created him never happened. Earth is still relatively normal, but Simon Petrikov/The Ice King is dead, having been pinned to the ground by the initial bomb of the Mushroom War in an otherwise successful attempt to stop it; humans aren't subject to Black Bead Eyes or The Noseless, and are still the dominant civilization; Finn is a farmer's son and now has said facial features along with, for reasons unknown, a simplistic robotic arm; Jake is a normal dog; and Marceline is an old woman (apparently, she never became a vampire, which was the reason for her eternal youth). Finn dons the Ice Crown to put out a fire but goes insane and causes the bomb to detonate, bathing Jake in radiation and turning him into a Lich.
"Puhoy" introduces another alternate universe called the Pillowverse.
Altum Videtur: The incantation for banishing/releasing Marceline's dad to/from the Nightosphere is "Maloso vobiscum et cum spiritum", which is roughly "evil be with you and with your spirit" in Latin. Other instances occur throughout the episodes.
Amazing Technicolor Population: Applies to a lot of the characters. For example, all the Marauders are a shade of blue, the cobbler is green, and Billy is grey. Justified; none of these people are human.
Gunter. Not even the crew is sure of whether... it's male or female. Of course, none of this is helped by the fact that the Ice King seems to have named every one of his penguins Gunter (or some variation thereof). It's hinted that it's a name he gives to anyone close to him while under the crown's influence.
Jake: Gunter's a WOMAN?!? Ice King: What?! No!... (lifts Gunter up and flips the penguin upside down)
"BMO Lost" has BMO & a bubble who sounds like a man taking care of a baby together &, at the end, BMO & Bubble decide to get married. The bubble is likely also genderless.
In Farmworld, Finn's baby sibling is naked, but of course we never see anything. Farmworld Finn only addresses the kid once, as "Younger Sibling."
Abracadaniel at first. He didn't want to kiss Princess Bubblegum, and he says, given the opportunity, he would "turn and push". He would even rather die than kiss her. "I'm going back to my cave to wait for someone to kill me." There are numerous more pieces of evidence like turning a wizard pink and turning Ice King's weapon into a butterfly, as well as looking remarkably like a set of male genitalia. The cherry on the sundae? He basically has the power over rainbows. He eventually does change his mind about kissing PB, though.
Huntress Wizard competed for Princess Bubblegum's kiss as well.
Both of the Lemongrabs. They live together, have no problem with hugging while they're butt naked, have many children, love each other very much, "keep each other company in lemony bliss," and insist that Princess Bubblegum "call first" before visiting. They refer to each other as "brother", which does not necessarily reduce the ambiguity.
Princess Bubblegum in "Sky Witch" has Princess Bubblegum deeply sniffing the shirt that Marceline gave her. And according to Maja the Sky Witch, it has tons more sentimental baggage than Hambo. Hambo is the sock-monkey doll Simon Petrikov gave Marceline when she became his adopted daughter after the Great Mushroom War.
Ambiguously Human: Pretty much every apparently human character in the show aside from Finn, to the point that 'Finn the Human' is his actual title. The only exception is Susan Strong, and even she isn't confirmed to be one, though it's pretty strongly suggested.
"This magic keeps me alive, but it's making me crazy. And I need to save you. But, who's going to save me?"
Amusing Injuries: Averted more often than not. When someone gets beaten up, it's treated quite seriously.
Averted in "No one can hear you" in which Finn has both his legs broken and Jake gets a concussion and both are knocked out. When Finn awakes (several months later) Jake's injuries and isolation have left him a little unhinged and Finn wears a cast to deal with his broken legs and is disturbed by Jake's delusions
Ancestral Weapon: Joshua, Finn's and Jake father, possessed a sword created from solidified demon blood. He promised in a series of holotapes that if the two completed a dungeon, it was theirs to keep. And so, Finn slew the giant one-eyed monster guarding the sword, making it his third sword.
Apocalypse How: The Great Mushroom War, the scale being Planetary Societal Collapse, or possibly Planetary Species Extinction. There is certainly a lot of evidence of a Post Nuclear Apocalyptic setting (like missile shells and such). If this is the case, then the War definitely caused a Depopulation Bomb. This is pretty much confirmed and gets some bits of Continental Physical Annihilation added in Season 4. In "Sons of Mars" we get a brief view of the Earth from space. There's a large chunk of the planet missing. Also, Princess Bubblegum's holographic diagrams in "Burning Low" shows a rotation of the Earth in the bottom left corner of the screen.
Arbitrarily Large Bank Account: The entrance to their house is filled with gold and jewels, as well as several other rooms in their house. Yet Lady Rainicorn's parents assume Jake is rich because he "Owns a human butler" not even acknowledging the hordes of treasure lying around.
The Ice King's Crown is revealed to be this in "Holly Jolly Secrets".
Marceline's Dad's amulet turns its wearer into a chaotic evil monster dedicated to ruling the Nightosphere with an iron fist.
Ascended Fanfic: "Adventure Time with Fionna and Cake" started out as a Gender FlipWhat If? comic by a character designer of the show, Natasha Allegri. Pen liked the drawings and decided to make an episode based off the idea. Her comic was itself inspired by Rule 63 art of the cast from drawthreads on 4chan's /co/ board.
Ass Shove: Beemo takes advantage of Finn's vulnerable tear-filled state to play Kancho on him. He used his whole hands instead of just his index fingers.
The criminal candy citizens who volunteer to live with Lemongrab in exchange for amnesty in "You Made Me," and are later tortured by him.
Lemongrab becomes this in "Mystery Dungeon," when a giant monster grabs him and literally squeezes all of the blood out of him as he cries for his mommy. He almost dies, too- but he did have it coming. He was this earlier, in "Too Young" when PB and Finn prank him so hard that he cries, but he wasn't quite as much of an asshole in that case, just annoying.
Astonishingly Appropriate Appearance: Most of the royalty have more human-looking appearances than the other inhabitants of their kingdoms. The most obvious examples are Princess Bubblegum, essentially a pink human woman who lives among much less humanoid candy creatures, and the Ice King, who is the only inhabitant of his kingdom not literally made out of ice or snow other than the penguins.
Ice King:FOOL! Your powers are no match for my magical crown! (Jake knocks off the crown.) Ice King:MY POWERS!
The giant ruby on the Snake-Armed Ruby Brainbeast's back.
Attractive Bent-Gender: Everyone in the gender-swap episodes, most notably Fionna, who, inexplicably, appears to be at least sixteen or seventeen years old instead of twelve or thirteen. Also, the Ice Queen, who looks about twenty-five years younger than the Ice King. Notably, she lacks the Ice King's enormous beard (which is often mistaken for a fat gut), is missing the long, pointy nose, and wears a beautiful blue gown instead of a loose, featureless blue robe. Averted with Lumpy Space Prince, who looks exactly like Lumpy Space Princess with a seedy teenage mustache.
Aura Vision: Princess Bubblegum creates a machine that can show peoples auras. Finn wants to see what Peppermint Butler's aura looks like, and he gets very nervous.
Lemongrab would be impulsive, psychotic, irrational and violent in the most frightening way possible when he is in a bad mood, and has even committed cannibalism.
The Ice King is mentally unstable and likes kidnapping people. However, he appears to draw the line at using lethal force on sentient beings, even though it's repeatedly shown that he's more than capable of it.
Several deranged and sadistic sociopaths, like Magic Man, Me-Mow and Ricardio fits very well to the trope as well.
Princess Bubblegum, on occasion. For example, in "Lady & Peebles". Not only does she go into a mysterious uncharted ice cave with Lady Rainicorn to save Finn and Jake, she goes through tentacle arms, laser eyes, and even beating Ricardio in hand-to-hand combat when Ricardio has GIANT ARMS AND LEGS. BY RIPPING THEM OFF.
Finn himself, naturally. He's an ordinary teenage human boy that regularly beats down monsters, wizards and eldritch abominations. Even with the help of a magic dog and some fancy swords, Finn himself has no special powers.
Beauty Equals Goodness: Seems to be a staple of the show, whether unintentional or intentional, as Finn and Jake are quick to label anyone as "evil" based solely on appearances alone. Marceline, while not exactly Good All Along, is nice to Finn and Jake after they impress her. With Ricardio, their assessment that he "looks evil" proves to be completely accurate. Often invoked, like in "To Cut a Woman's Hair":
Jake: If she [the Tree Witch] was good, she wouldn't be so ugly...
Except when the show makes exceptions, such as with the Guardian Angel and the Dimple Plant.
Once again played straight with the Fruit Witches in "Dad's Dungeon".
Flame Princess is a teenager who does seem fairly innocent. However, even her own father calls her evil; and boy, does she show it. Finn, meanwhile, insists that "She ain't evil. She's passionate."
Ice Queen from "Fiona and Cake". Physically, she's much more attractive than Ice King; but while the Ice King has tinges of comedy in his attempts to kidnap Princess Bubblegum, Ice Queen just ends up storming the castle, launching a giant ice spike to break the wall.
Be Careful What You Wish For: A major theme of the "Farmworld Trilogy". Due to unforeseen results of the wishes he grants, Prismo, an all-powerful being inhabiting the Time Room, actually points out to Finn and Jake that his wishes tend to have ironic twists. Finn is Genre Savvy enough to realize that simply stopping the Lich's current scheme is not adequate, since the Lich could simply try a new plot, so he wishes for the Lich to never have existed. This creates a timeline where Simon Petrikov froze the bomb that kickstarted the Mushroom War and created the Lich, resulting in (without going into the details) Finn donning the Ice Crown and going insane, causing the bomb to go off and turn (normal dog) Jake into the Lich. In an attempt to avoid Jake's wish from screwing up, Prismo basically tells Jake what to wish for—retroactively changing the Lich's wish, which solves all immediate problems, at least for the moment.
Bedsheet Ghost: In "Too Young", Finn and Princess Bubblegum stage a 'prank' on Earl Lemongrab by dressing up as bedsheet ghosts, running up to Lemongrab and punching him in the stomach.
The Beforetimes: Source video tapes that start with a reference to the mysterious FBI.
There are times when Princess Bubblegum shows she's not as innocent (or sane) as she appears, like when she told Finn about the Paralysis Potion she made for no observable reason.
Likewise, Finn finds out Marceline is a lot nicer than she seems on the surface.
Benevolent Genie: Prismo. He's so nice he even admits that all his wishes have a catch, and walks Jake through getting the best possible outcome for himself and Finn. When Jake, at first, wants to waste his one wish on a sandwich, Prismo just makes him the sandwich (then conjures up a jar of pickles for good measure) and allows him to save his wish for later.
Beware the Nice Ones: Aside from the fact that both Bubblegum and Finn can get a little... iffywhen provoked, apparently the possessor of the most evil soul in Ooo is a Gunter. And sure enough, one later almost takes over the Candy Kingdom on its own...
Bi the Way: Jake has a secret crush on Billy. Wonder what Lady Rainicorn would say...
Anything Lady Rainicorn says, since she speaks in Korean.
In "My Two Favorite People," this exchange between Lady Rainicorn and Jake after Jake asks her if she knows a joke:
Lady Rainicorn [in Korean]: I can't think of one... but remember the time when we ran naked through that farmer's cabbage patch? (Giggle) He was so offended! Finn: What's the joke? Jake: Uh... well, the joke doesn't... translate very well...
In the end "From Bad To Worse", Lady Rainicorn reveals that she didn't free the zombified Jake due to Idiot Ball.
Black Bead Eyes: The standard design of most characters' eyes in the main continuity, including Finn. Human characters in the "Farmworld" timeline have more realistic eyes. It is unclear whether this is simply a different design aesthetic or an indication that most humanoid characters in the main continuity (like Finn, Bubblegum, Marceline, etc.) really do look different from "regular" humans due to mutation.
Black Comedy: The show does touch on some relatively dark themes, but never quite takes itself seriously. Especially in Season 4 and parts of Season 3. The most egregrious examples of this are "Princess Monster Wife", "Holly Jolly Secrets", "No One Can Hear You", and Princess Bubblegum's failed science experiment, Lemongrab.
Many of the Magic Man's spells. They tend to involve turning sentient beings into body parts or inanimate objects, fusing people together or turning them bald.
Most of the Lemon Children are grotesque, deformed creatures, including some that appear to be little more than sentient piles of goo. The Lemongrabs love them all the same, though, and are ever compelled to create more of them.
Book Ends: "Food Chain" begins and ends with Candy children playing.
Jake: Think happy thoughts - little cute bees, little babies, tiny, tiny bunnies...
Brainwashed and Crazy: The Ice King. As is revealed over the course of the series, his crown is the source of all his power, granting him extended life and control over the forces of nuclear winter, but destroys his connection with reality.
In the "Graybles" episodes, Cuber acts as host and talks directly to the viewer.
"See if you can guess the theme by the end of the episode, okay?"
In the Adventure Time game Hey Ice King, Why'd You Steal Our Garbage!!?, Jake makes a comment about finishing the game right before going up against Ice King - who refers to himself as 'the final boss' a moment later.
Breather Episode: "Box Prince", which is a relatively "normal" episode in the middle of an action-packed and progressively arc-driven season, to the extent of feeling like a throwback to earlier seasons.
'Mind Games', by 'J. T. Dawgzone'. In "Reign of Gunters", Finn mentions reading this book: Jake is appalled and replies that the book is "really bad advice" and should only ever be read "for laughs". In the next episode, "I Remember You", it turns out that the book is the Ice King's chief reference on 'how to get ladies'.
In "Ricardio The Heart Guy" when Finn tries to impress Princess Bubblegum, they start talking about Zanoits, and he asks if they're good things or not. In Ricardio's other episode, "Lady and Peebles", after Finn recovers, Bubblegum informs him that Ricardio poisoned him and Jake with Zanoits.
In "BMO Noire", Finn and Jake go to a party, leaving BMO alone. When they return, Jake has lipstick all over his face and an arrow in his head, and Finn is carrying a dead Sea Lard. It isn't until nearly 30 episodes later, in "Princess Potluck", that we see the other side of this.
Princess Bubblegum is eccentric, and a bit Cute and Psycho, but a goodCute Psycho. However, she cares deeply and personally about her citizens, to the extent of considering herself their mother in more ways than one, and will do anything to protect them. Just don't piss her off.
When characters call on Finn to be a hero they also deal with the fact that he's a little boy and when they deal with him as a little boy they also must account for his heroic status. This is much less of a factor for the more mature Fionna. (Who actually wears bunny ears.)
Canis Latinicus: Magic often seems to work this way, such as with the Donut Witch from "Witch's Garden" and Ash from "Memory of a Memory". Averted with Joshua and Flambo.
Can't Have Sex, Ever: Finn and Flame Princess' first kiss creates an emotional reaction within Flame Princess that causes her to melt through the earth and almost leads to the end of the world. Not to mention that fact that merely touching her sears Finn's flesh (unless he is enchanted with Flame Protection). So she and Finn just stick to kissing a lump of coal instead as a stand in for each other.
Cargo Envy: Jake has a hard time lampshading this trope. He eventually just explains himself straight-up:
Jake: I bet you wish you were my butt. Finn: ...What? Jake: I bet you wish you were my butt, because then the Princess would... no, wait... I bet you wish you were the Ice King and the Princesswas my butt! Finn: What?! Jake: Hmm... OH! I bet you wish Bubblegum would kiss you, too, like the Ice King and my butt!
The Candy Kingdom is populated by candy and other sweets. Lampshaded in "Videomakers", where Princess Bubblegum is shown drinking a sentient smoothie. In another episode, Bubblegum is stress-eating a sentient ice cream cone. The Hyoomans were intent on eating the entire Candy Kingdom (inhabitants included) before being driven back, and the Earls of Lemongrab and their Lemon Children attempt the same in a later episode.
Peppermint Butler shows an interest in eating Finn and Jake.
Rainicorns apparently love the taste of human, but since they are so rare, they eat Soy People. Finn also thinks Soy People are tasty. "They say you can hardly tell the difference!"
Wildberry Princess often makes meat pies, being a fruit and all.
While visiting the Duchess of Nuts (an acorn-person, of course) with Finn, Jake had already eaten some nuts in a bowl before realizing the potential harm he may have caused.
In "Hitman", Jake serves himself some meat which he says came from someone called 'Meat Man'. Finn begins to question if it's okay to eat that, but Jake just shrugs him off. Later, we see Jake remorsefully mutter in his sleep on the matter.
Jake: I had a dream about Meat Man. I think I'm gonna stop eating Meat Man.
After Lemonjon dissolves himself into non-living lemon candy, Finn has no problems partaking of his remains.
Olivia Olson as Marceline displays her vocal skills in multiple episodes.
The Gender Bending episodes seem built around the idea of having the guest cast sing. Madeline Martin voices Fionna the Human and sings in both episodes, with NPH-as-Prince Gumball in "Adventure Time with Fionna and Cake" then Donald Glover who voices Marshall Lee, sharing a duet and singing to Fionna in "Bad Little Boy".
Cerebus Retcon: In the original short, Abraham Lincoln appeared as the king of Mars. At the time, this was just the height of Surreal Humor. In "Son of Mars", it's revealed that this is actually supposed to be the same person as our Abraham Lincoln, and the reason he's dead in our universe is because he traveled back in time and was killed there.
The third season and onward is notably darker and more mature (and less silly) than the previous seasons.
BMO's character arc has really taken an...interesting direction. Initially, BMO was developed as a comedic character to play off of Finn and Jake. As the series progressed, it became more aware of its limitations and tried to openly defy them. BMO's dream is to be a "real boy" and it talks to itself in the mirror regarding the topic. Episodes like "BMO Noir" and "BMO Lost" are also major indicators that it is something more than meets the eye. In them, BMO surrounds itself with strangely emotional characters such as Lorraine, Ronnie, Bubble, and Sparkle, and throws itself headlong into quests, as if it wishes to become a hero like Finn. Even if it would most likely die, it still continues headlong in danger, often breaking down physically or mentally. "James Baxter the Horse" has BMO caring for an egg, pretending to be pregnant... and then BMO drops and cracks the egg. BMO, of course, cries, likely thinking the egg was really a baby. The concept evokes all kinds of Fridge Horror.
At least one recurring character has been permanently killed off (as far as we know) in every season from 4 onwards.
Finn is ordered to kill an unaligned ant, which he refuses to do.
Hunson Abadeer's amulet "fills the user with chaotic evil".
Finn asks the Flame King if Flame Princess is necessarily evil or if she might be "Chaotic Neutral." Flame King says his daughter is definitely evil but agrees with Finn that the right guy might be able to change her alignment. Though she would suffer experience point penalties for acting out of her alignment.
Check Please: "Videomakers" demonstrates that, despite apparently watching tons of movies from our time, none of the characters have learned its proper usage. Cartoon Network recently started running an ad for an Adventure Time "mix your own adventure" video creator with two strange, but coherent examples, and then this:
Ice King: I've soiled my tunic!
Jake: Mmm, check please.
Chekhov's Boomerang: The Enchiridion. In its first appearance it's just a "hero's handbook," but then the Lich acquires it. It proves to be perhaps the most powerful artifact in the Adventure Time canon; when combined with various gems guarded by the princesses of Ooo (and the Ice King), it has the ability to open a portal to any of several parallel universes.
Chekhov's Gun: At the end of "I Remember You", a flashback shows Ice King giving a young Marceline a teddy bear, which happens to be the same teddy bear that her ex-boyfriend sold in "Memory of a Memory". This is turn becomes important in the episode "Sky Witch".
Chekhov's Gunman: The snail. Appears in every episode as a minor Easter Egg. Releases the Lich from his prison and becomes his vessel after he is defeated.
The people of Lumpy Space (especially Lumpy Space Princess) seem to abandon all shreds of sanity when they "turn lumpy".
Tree Trunks attempts to have tea with and seduce monsters (Tentacle monsters) when they attack her. Finn and Jake seem to consider Tree Trunks's idiosyncrasies to be due to senility, however.
Ice King later turns out to be a rather horrifying deconstruction of this trope. His strange behaviour actually stems from having had his mind warped and his memory erased by an Artifact of Doom. The process was slow and extremely painful, destroying his relationships with his fiancee and surrogate daughter, and it's implied that most of his actions are a subconscious attempt to replace them. By the time the main series takes place, he's barely a shadow of his old self and is miserable due to his inability to form normal relationships. And, it's implied that without the mental remnants of his old self restraining him, he would be just a violent madman rather than an ineffectualCloud Cuckoo Lander. When this control occasionally slips, he is much moredisturbing.
Comic Book Adaptation: KaBoom started putting out monthly comic books starting 2012. This marks the first time a comic book based on a Cartoon Network show is put out by a publisher other than DC Comics.
Continuity Cavalcade: Many characters from past episodes return in both parts of "Holly Jolly Secrets".
Played straight in the episode "Jake Suit", in which Finn and Jake not only walk around in the Fire Kingdom without Flame Shield, but Finn gets lowered into a volcano directly above the surface of the lava and only gets a singed hat.
Cool Horse: Lady Rainicorn and Lord Monochromicorn.
Cool Old Lady: Tree Trunks and the old woman who get Finn and Jake to return to beating up evil.
Cool Plane: Finn's paper airplane skills are something to behold.
Finn: I'll fly the paper as an airplane down the bedroom ladder. It'll triple barrel roll past the kitchen open the fridge and cook some eggs then eat the eggs and unfold itself as it lays itself in front of Marceline's door.
Finn and Jake's family sword is made out of freaking DEMON'S BLOOD.
"The family sword. It's made out of babies!"
The grass sword. It has a curse on it that binds it to Finn at all times. When it's not being used as an actual sword, it takes the form of a grassy armband around Finn's wrist. Fast, deadly, and always on-hand.
Crapsack World: Ooo a post-apocalyptic land full of magical monsters ruled by royalty that range from innocent-but-crazy to outright evil. It was even worse during Marceline's childhood, which is implied to have been during or shortly after a world-ending nuclear war.
Creating Life: Appears several times throughout the series, and many characters originated in this manner.
Princess Bubblegum creates both Lemongrabs in her lab. Lemongrabs 1 and 2 later use the same technique to create the Lemon Children. It is implied that Bubblegum created the entire Candy Kingdom.
In "Goliad", Princess Bubblegum creates her successor, Goliad, using 'a fairly standard candy creature soup' as well as her own DNA. When Goliad learns to rule others by force, Princess Bubblegum creates Stormo to counter her using Finn's DNA.
NEPTR is created by a combination of Finn's crude engineering, a bolt of lightning and Ice King's magic in "What is Life?"
Gingerbread man versions of creator Pendleton Ward, storyboard artist Adam Muto, and creative director Pat McHale can be seen in the background in one scene in "The Enchiridion!". Also, Gingerbread Muto can be seen in the Title Sequence (and Gingerbread Rebecca Sugar (Adam Muto's storyboarding partner) for the opening sequence to the Gender Flip episode "Adventure Time with Fionna & Cake").
Several episodes feature a stubbly, bespectacled face representing character designer Phil Rynda. The face shows up on minor characters and as the symbol used to summon Marceline's dad, among other instances.
Creepy Child: Marceline as a kid. "I'm hurting you because I love you."
Crying Little Kid: How Simon and Marceline met. He found her crying in the ruins of a city during the aftermath of the Mushroom War.
Cue the Sun: In "Evicted!" to symbolize that things will start to go right for Finn and Jake. For a while, it doesn't.
Finn: Jake, get the camera! Jake: I can't! I'm paralyzed by the cuteness!
Cuteness Proximity: In the episode "Gut Grinder", villagers found Jake to be cute and adorable, showing their affection by heavily petting him.
Cutting the Knot: In "Dad's Dungeon", Finn encounters a monster with a key on a chain around its wrist, obviously meant for the lock on the family sword. He simply breaks the lock through sheer force and slays the monster with the sword.