Calvin having to get a shot, while his mom hides her face in embarrassment:
Calvin: Is that a shot? Are you going to— AUGH! IT WENT STRAIGHT THROUGH MY ARM! OW OW OW OW OW!!!! I'M DYING! I HOPE YOU PAID YOUR MALPRACTICE INSURANCE, YOU QUACK! WHERE'S MY MOM!?
In one strip, Calvin yells to his mom from across the house. She tells him to walk over to where she is. He does... and tells his mom "I stepped in dog doo. Where's the hose?" Topped off in the Anthology edition as Watterson's comment is simply "Right lesson - wrong time."
At the beginning of the baseball arc, Calvin realizes he's the only boy on a playground full of girls, and starts freaking out about being in a "Cootie central." Susie retorts with "Relax. Stupidity produces antibodies."
Calvin's ransom note to Susie. "Dear Susie, If you want to see your doll again, leave $100 in this envelope by the tree out front. Do not call the police. You cannot trace us. You cannot find us. Sincerely, Calvin."
Made funnier by the panel break before the last sentence, causing a beat when reading.
Most of the "Dad polls" strips. For example:
Calvin: (Looking at an old yearbook) "Is this you with the keg and the 'Party Naked' T-Shirt?"
Dad: "Give me thaaaaaat!"
Calvin: "Who's the bimbo with you in this old prom picture?"
The strip in which Calvin employs Loophole Abuse to answer the question "Explain Newton's First Law of Motion in your own words." His answer? "Yakka foob mog. Grug pubbawup zink wattoom gazork. Chumble spuzz."
Calvin's little monologue about the weirdness of cow milk was hilarious enough, but it was even better in the 10th Anniversary Collection:
Calvin: Who was the first guy who looked at a cow and said "I think I'll drink whatever comes out of these things when I squeeze 'em!"?
Bill Watterson: It's sometimes frightening where my mind will go if I let it. Who was that guy?!
Unfortunately, many people like to ruin the joke by pointing out that the guy who discovered cow milk most likely saw a cow nursing its calf and decided that cow milk was safe to drink.
"I hope you suffer a debilitating brain aneurysm, you freak!"
Three panels of a hideous space alien contemptuously addressing a classroom and proceeding to demonstrate his planet's destructive technology, one panel of Calvin pulling a hideous face at the front of the classroom growling gibberish. "Miss Wormwood, shouldn't he be in some kind of special school?"
This part of the Mercury report arc:
Susie:(to Calvin) Look, bird brain, you wasted this entire week in the library. We have to give our report on Monday. You'd better bust your butt over the weekend, or I'm telling the teacher you didn't do any work. Got it? ...WELL, WHAT DO YOU SAY?! AM I GETTING THROUGH TO YOU?? THIS IS IMPORTANT!
(the next panel has Spaceman Spiff and an angry alien in place of Calvin and Susie)
Alien: Gronk! Gribble gok! Gak gork! Goonk!
Spaceman Spiff: Our hero regards the strange alien. ...It seems to be trying to communicate.
The whole story arc. Susie takes the report so seriously that she thinks getting into a good college depends on her doing well on it. Calvin, of course, couldn't care less.
This argument between Calvin and Hobbes:
Calvin:(after Hobbes had called him a liar) ...Well, you're just a poop head! So there! THBPBPTHPT!
Hobbes: POTTY MOUTH! POTTY MOUTH! CALVIN IS A POTTY MOUTH!
Calvin: You're asking for a toothless mouth, buster!
Hobbes: Yeah? Says you and what army? You couldn't knock the teeth out of a mosquito!
Calvin: Ha! Mosquitoes don't have teeth! That shows how dumb you are!
Hobbes: Compared to you, I'm Einstein! Leggo my leg!
Calvin: Ow! Go stick your nose in a rubber hose, you walking flea condo!
Hobbes: I say it takes one to know one, bozo! Why don't you go play in the food processor!
What makes this funnier is the whole argument was over a croquet game.
"TEN MILLION BOTTLES OF BEER ON THE WALL, TEN MILLION BOTTLES OF BEER..."
One strip, Calvin is pounding nails into the coffee table. Mom runs in screaming "CALVIN! WHAT ARE YOU DOING TO THE COFFEE TABLE?!?" After a Beat Panel, he looks up and says, "Is this some sort of trick question, or what?"
The strip where Calvin's dad comes home to an army of snowmen lined up and saluting him, much to his chagrin:
Calvin's Dad: He knows I hate this.
The strip where they talked about the new year in relation to the snowmen that Calvin made was both serious and funny.
And then there was this one strip where Calvin used Forced Perspective to freak out his dad (Calvin built half a giant face and fingers on a hill top so it looked like a giant snowman was peeking out from over the hill)
Not to mention the strip where Calvin built a crowd of snowmen crossing the road, complete with a "Snowmen Crossing" sign. ("CALVIN, I'M LATE FOR WORK!!")
Calvin: I have no idea. Now I wish I'd watched that dumb show! Maybe they stabbed him with an icicle.
Pretty much anytime either parent finds Hobbes somewhere unusual in the house while Calvin's at school (usually so Hobbes can ambush Calvin).
In one arc, Calvin tries to wheedle out of a school assignment by convincing Hobbes to write his paper for him, giving him specific (albeit poor) instructions. Hobbes agrees, but in true trickster spirit, completely ignores Calvin's instructions and writes the report on his own whims. What makes this funny? At the end, Calvin is furious at Hobbes' prank only to discover that the report was great, and got an A+. (possibly Calvin's only good grade ever), and the arc ends with Calvin still trying to be mad at Hobbes for the principle of the thing as Hobbes gets starry eyed over a possible journalism career.
To elaborate, the report was to write and illustrate a story. Calvin decides to get out of it by going forward in time with Hobbes from 6:30 to 8:30, when he will have already written the paper. When that doesn't work, Calvin teams up with the 8:30 Calvin and they both travel back to 7:30 in an attempt to force that Calvin to do the homework. To put it simply, things end up escalating into a Mêlée à Trois. Meanwhile, the 6:30 Hobbes and the 8:30 Hobbes write Calvin's story and give it to him when he returns, resolving the whole mess. What they don't mention is that the story was about how he tried to avoid doing the assignment.
One strip has Calvin throw a snowball at Susie, misses, then comes back with a shovel full of snow,and chases Susie.
Susie's attempt to get Calvin to play house, including his refusal to accept her rabbit doll as their "baby." What really makes it work it that it's all drawn in the style of a serious, soap opera comic.
The rest of that story arc counts too. Calvin is sick of being a kid and wants to be a tiger instead, so he dresses up as a tiger and goes out into the forest with Hobbes. At one point he reads that tigers are "secretive". They get into an argument, then Hobbes promises to give Calvin a hint.
Calvin: Ok, shoot!
Hobbes: The flea market.
Calvin: THE FLEA MARKET?! WHAT KIND OF LOUSY HINT IS THAT?!
Hobbes: Do you know how your parents got you?
Calvin: I was...what? What are you saying?
Hobbes: No more hints.
Hobbes gives Calvin a haircut that quickly goeshorribly wrong. Bill Watterson went on record saying that he rarely laughs when he draws, but drawing the results of Hobbes' work actually cracked him up.
"...Our first president was NOT Chef Boy-Ar-Dee, and you ought to be ashamed to have turned in such preposterous answers!"
At one point Calvin is told to stop making a particular face, as it will freeze that way if he does so. This kicks off a week of strips containing the following:
Panel 1: *Calvin walking about with zombie-grimace face* Panel 2: *approaches Hobbes, who jumps in surprise at the sight* Panel 3: *beat panel as Hobbes considers Calvin* Panel 4: *Hobbes adopts a similar expression and falls in with Calvin* Hobbes: *thinking* "When in Rome..."
Then right afterward, he and Hobbes make the face to Susie, and through her point-of-view, you just see Calvin standing there saying "Hi, Susie" with Hobbes as a stuffed tiger sitting next to him.
Later, Calvin's face does freeze, and he comes to the dinner table looking like that. After failing to explain why his face can't return to normal, he throws on a sheet with eye-holes proclaiming "Look, ElephantMan!"
Calvin: Cigars are all the rage, Dad. You should smoke cigars!
Calvin's Mom: Flatulence could be all the rage, but it would still be disgusting.
Calvin and Hobbes are at Susie's birthday party, and the birthday girl is passing around paper plates. Calvin confids in Hobbes his hopes that the cake isn't something gross, like coconut. Hobbes assures him that it's chocolate.
Susie (Off-panel):HEY! WHO CUT A PIECE OF MY CAKE ALREADY?! I DIDN'T EVEN GET TO BLOW OUT THE CANDLES!!
Hobbes, while Calvin Face Palms: It's nice and moist, too.
In one sunday strip, Calvin and Hobbes are outside walking through the snow;
Calvin: I asked dad if he wanted to see some New Years resolutions I wrote. He said he'd be glad to, and he was pleased to see I was taking an interest in self-improvement. I told him the resolutions weren't for me, they were for him. That's why we're outside now.
Hobbes: I wondered what the rush was.
An entire Sunday comic is devoted to Calvin's dinner reciting Hamlet to him. It then, after a short pause, begins singing. Calvin eats it before it can do anything else.
In the last panel, his mom remarks happily about how quickly he finished eating, and with a look of supreme disgust Calvin says "Let's never have that again."
When Calvin tells his mom he DOES have common sense when he's told he lacks it, he turns to the reader and says happily "I just choose to ignore it!"
In one arc, both Calvin and Susie have been sent to the principle's office (Susie was passing notes, and Calvin basically announced it for all the world to hear). They become worried that they may be spanked, and when Mr. Spittle comes to talk to them, they start freaking out, culminating in, "WAAHHHH!! I WISH WE WERE DEAD!!"
Mr. Spittle:(Thinking, giving Aside Glance) I hate this job.
Calvin and the multiple duplicates agree to split up going to school. When Duplicate #5 is asked to demonstrate a problem assigned the previous day, he insists he wasn't there.
Miss Wormwood: Yes, you were, Calvin. Didn't you do your problem? Duplicate #5: I'm not Calvin. I'm Duplicate Number Five. Duplicate Two was here yesterday, not me. We're all taking turns. Number Two will be back next week, and you can ask him then. (later in Mr. Spittle's office) Duplicate #5: Look, I don't see what's so hard about this!
The retelling of Calvin's favorite book, Hamster Huey and the Gooey Kablooie as told by Calvin's dad, frustrated over having to read it night after night. While we never get hear the story, we see Calvin and Hobbes lying in bed afterwards with their eyes as big as dinner plates.
Calvin: Wow, the story was different that time!
Hobbes: Do you think the townsfolk will ever find Hamster Huey's head?
Even funnier in the Anniversary edition, where Watterson mentions that, after that arc, he learned far more about Bats than he'd ever cared to know.
After Calvin gives Susie her Valentine, she's furious.
Susie: Calvin you baloney brain! You sent me a hate-mail Valentine and a crummy bunch of dead flowers! So here's a Valentine for you, you insensitive clod! *nails him point blank with a snowball and walks away, smiling* 'A Valentine and flowers! He likes me!'
Calvin, buried in the snow: 'She noticed! She likes me!'
The entire exchange about Calvin being called "Boy of Destiny".
Miss Wormwood: Here is your paper, Susie. Very good. Here is yours, Calvin. By the way, you can stop signing your paper "Calvin, Boy of Destiny", and I think your time would be better spent studying than drawing "official notary seals" at the bottom.
Susie: Everyone I know thinks your destiny is a private cage in the primate house.
When Calvin secedes from his family to go to the Yukon, he changes his mind after roughly a morning spent in the woods, leaving Hobbes behind in his rush to get back home. His parents go out there after dark to look, and his Mom actually calls out Hobbes' name. The sheepish look on her face coupled with his dad's reaction seals it.
Dad: I may be crazy, but I'm not as crazy as you.
One where Calvin notices a door that he had never seen before in the hallway. He opens it, to find a gigantic Muppet-like version of his mom offering oatmeal. Calvin immediately panics, knowing this to be off, while "Mom" keeps trying to offer the oatmeal.
Calvin: What's going on?! This isn't my house and you're not my mom! (two panels later) Auugh! I'm trapped in a lab and they're trying to make me imprint on my species before releasing me back to the wild!
*Cue outside shot of a large scale model of Calvin's house in a cage, and two aliens in lab coats, one with the Mom Muppet*
Alien with Muppet: He's onto us Wayne.
Wayne: There goes our funding.
*Calvin wakes up, heads to the table, where Mom is bringing him a bowl of oatmeal, wearing the same outfit*
Mom: Morning. Here's your breakfast. What's wrong?
Calvin & Hobbes are walking through the snow when Calvin begins ranting. Keep in mind that everything Calvin says here, except for his last line, is spread over three panels before Hobbes finally gets a word in.
Calvin walks by Mom, proclaiming himself to be her "big accomplishment in life". Cut to Mom talking to Dad: "I'm depressed."
One of the more bizarre "Calvin's dinner comes to life" gags, this time reciting Hamlet Act III Scene 1 ("To be, or not to be"). It then stabs itself with a fork and "dies", leaving Calvin to wonder what just happened. It promptly comes back to life and starts singing "Feelings". Calvin eats it right away just to shut it up.