Futurama occasionally showed flashback-based, almost art-house style episodes.
They also did two "Treehouse of Horror"-style episodes ("Anthology of Interest"), meant to be a yearly thing.
"Naturama", which is done in the style of a wildlife documentary with the cast portrayed as talking animals.
"Reincarnation" and "Saturday Morning Fun Pit" depict the characters and their adventures in the styles of other types of animation (rubber-hosed 1930s cartoons, 8-bit video game, an anime, and parodies of popular American cartoons, like Scooby Doo, G.I. Joe, and The Smurfs)
There's also "Blackfoot and Slim", which is an episode presented as a nature documentry that observes Dexter (or Blackfoot as he is called in this episode) and his daily activities until he realises what's going on and tries to run away from them until he is shot with a tranquilizer dart.
"Framed", a completely normal Slice of Life story about Dexter's broken glasses starting a trend at school. It's very dialogue-heavy and introduces a bunch of characters that are never seen again.
"Star Trek": a Film Noir parody about Steve's rise to stardom after writing a book about Roger.
"Tear Jerker" and "For Black Eyes Only": full-episode James Bond parodies
"Blood Crieth Unto Heaven": a parody of the play "August: Osage County" with Patrick Stewart shown in live-action
The ass-kicking Christmas episodes they've done, such as "The Most Adequate Christmas Ever," "Rapture's Delight," "For Whom the Sleigh Bells Toll," "Seasons Beatings," and the temporarily Banned Episode "Minstrel Krampus."
"Hot Water": A Musical Episode featuring a killer hot tub. Was meant to be the final episode of the series as the writers were afraid that American Dad would be canceled, but when news hit that it was renewed, they put this as a season seven premiere and wrote off the deaths as non-canon.
"Lost in Space": A Day in the Limelight for Jeff and a completely straightforward (except for a few jokes) science-fiction story.
Avatar The Last Airbender had "Tales of Ba Sing Se," in which the episode was divided into a series of four-minute short stories, highlighting different characters and their individual adventures in the city of Ba Sing Se.
Not to mention the lampshade-happy "Ember Island Players", where the gang watches their adventures as performed by a (misinformed) theater troupe.
Transformers Armada had an episode that focused entirely on the Decepticons. Rad, the usual narrator, was replaced by Sideways, and the Autobots got barely any screen time, with the kids entirely absent.
The three-part Star Wars parody episodes: "Blue Harvest", "Something Something Something Dark Side", and "It's a Trap!".
"Brian and Stewie": A Bottle Episode based on the All in the Family episode "Archie in the Cellar" where Brian and Stewie (meaning that it's Seth MacFarlane talking to himself for 22 minutes) are locked in a bank vault for an entire weekend and play off each other.
The season nine premiere "And Then There Were Fewer" is a feature-length episode (and the first episode to be shown in high-definition) presented as an almost straight Agatha Christie style murder mystery, in which several recurring characters are bumped off permanently.
"The Man Who Killed Batman", where a nobody mob underling, "Sid the Squid", acts as the lookout on a job in which Batman is apparently killed. From Sid's perspective, Bats fell into a gas explosion, but everyone else thought Sid pushed him. He has to deal with a jealous Joker, an obsessed Commissioner Gordon, and a few rival mob bosses who can't believe his story. The producers admitted that this episode was an experiment to see if they could make a good episode with Batman appearing as little as possible (indeed, Batman only appears for about three minutes total in this episode). Arguably, they succeeded.
"I've Got Batman in My Basement", about two kids who take in an unconscious Batman and outwit the Penguin Home Alone-style. Unlike the others, this one was poorly received by fans.
Aquaman's Outrageous Adventure! relegates Bats to a supporting role and instead focuses onAqua Man as he (begrudgingly) takes his family on vacation only to sneak away at any opportunity to help other heroes.
Chill of the Night! features a much darker plotline with many shout outs to TAS, a darker style of animation and an in depth look at Batman's origins (the show had previously stayed away from even showing Bruce without the mask, much less exploring his personal history).
The Knights of Tomorrow! shows a possible future for Batman and features the animation debut of Damian Wayne, the current Robin.
Bat-Mite Presents: Batman's Strangest Cases! was hosted by the 4th wall breaking Bat-Mite and showcased three bizarre interpretations of the Bat (all voiced by actors other than series star Diedrich Bader who voices a supporting part in one segment):
Bat Boy and Rubin, based off of the MAD parody of the same name, trying to solve a series of strange murders.
A segment based off of the manga version of Batman, done in a style similar to the Speed Racer cartoons. Features Batman and Robin facing off against "Lord Death Man" not long after he was introduced in the American comics by Grant Morrison.
Bold Beginings! is told almost entirely in flashback as Aqua Man, Green Arrow and Plastic Man exchange stories about the first time they teamed up with Batman while waiting to be rescued for Mr. Freeze. Almost a reverse of "Almost Got 'Im" from TAS.
Four Star Spectacular! is four shorts starring Adam Strange, The Flash, 'Mazing Man and The Creature Commandos with Batman only making cameo appearances.
In Teen Titans the series finale, "Things Change", kept the fight with the Monster of the Week to a small B-plot (and never did get around to resolving it), while most of the episode focused on Beast Boy trying to reconnect with a girl who may or may not be Terra. While the series always dealt with emotional issues, usually a layer of superhero metaphor was applied. Here it was simply, bittersweetly Slice of Life.
The final season had a couple of these. Another episode dealt entirely with Raven babysitting a trio of superpowered toddlers, and another was about the HIVE Five trying to capture Kid Flash, and Jinx's High Heel Face Turn.
"The Wizard Of Odd" didn't feature any kind of project from Phineas and Ferb at all, instead focusing on Candace as the center of a The Wizard of Oz-type plot.
The Sealab 2021 episode "7211" is basically free of humor, because the Sealab 2020 episode on which is is based (in which the crew works fast to save another submarine from nuclear meltdown) is left mostly unchanged, merely trimmed for time and re-dubbed in the 2021 voices. In the context of a series filled with non-kid-appropriate jokes and a crew that never gets its act together, the effect is Anti-Humor.
The Danger Mouse episode "Once Upon A Timeslip" takes the cast to Medieval days to enact a Robin Hood story at the behest of the announcer and a quirky microphone. DM lampshades it early on:
DM: The announcer's voice is controlling the picture, dictating what we do!
Penfold: Crumbs, chief. What are we gonna do?
DM: Dunno...have a dafter adventure than usual, I suppose.
Adventure Time gave us "Five Short Graybles" (five loosely-connected shorts with the unifying theme of the five senses) and the ever-popular episode "Fionna and Cake", which Gender Flipped the entire cast.
The episodes that focus on Marceline and Simon Petrikov (before he became the Ice King).
One episode of Martha Speaks "Return of the Bookbots: the Case of the Missing Words" was an extended episode of the Bookbots, a potential TV series Martha had pitched back in "the Martha Show pt. 2". None of the regular characters appear outside of their Bookbot forms well, at least not until the end, anyway.
Animaniacs has an episode that mixes all of the skits' different characters together, making for such hilariously awkward combinations such as "Mindy and the Brain," "Pesto and Runt", and "The Warner Bros and their sister Slappy."
Every episode of Sheep In The Big City ends with a rant from the Ranting Swede, except one that didn't. "Due to a scheduling conflict, we now present The Ranting Norwegian..."
"Darkwing Doubloon" portrays Darkwing and his supporting cast as daring swashbucklers out to stop a dreaded band of pirates as played by the Fearsome Five.
"The Secret Origin of Darkwing Duck" features a framing device set in the far off future where an old janitor who may or may not be Darkwing's ghost telling two kids resembling Gosalyn and Honker his true origin, in which he was sent to Earth as a baby after his planet exploded.
"A Star is Scorned" takes place in the real world with Darkwing and the rest of the cast as actors and themselves at the same time with Darkwing trying to keep his role as star of the show.