For moon watching, Achakura makes Nagato's apartment look like the surface of the moon... causing her to become very, very small. When Nagato returns home, the alien is mesmerized by Achakura's tinier form. Then Kimidori-san sells her for 300 yen.
Mikuru and Tsuruya admire Haruhi's Picasso-esque chocolate bust, only for Haruhi to explain that she meant to make one of Kyon. Yuki tells Haruhi that it has an accuracy rating of 99.8%, but for reasons incomprehensible to people who rely only on sight.
Yuki makes sculptures that look exactly like unmelted chocolate bars. Tsuruya and Mikuru declare this amazing, while Haruhi says she finds it amazing how pointless the feat seems.
From the very first episode, sheep Mikuru ("Teeter totter, teeter totter.") and sheepdog Haruhi trying to get her to jump through a flaming hoop so Kyon can count sheep to fall asleep.
Episode 12, the episode where Kyon and Itsuki get all dolled up as hosts for Valentine's Day, has this gem:
Itsuki: ... and I'm Itsuki, the Crimson Super Ball. Guess why they call me that. Hollaaaa~
"Hi, I'm a hawk."
"No way! You're a crane!"
Tsuruya's unexplained beef with Mori.
Kyon and Itsuki are about to finish a board game when a battling Tsuruya and Mori-san blast through, scattering the game pieces everywhere. All they can do is response is laugh helplessly with tears in their eyes, as the wind from the battle blows their hair and clothes dramatically.
From Churuya-san:
Churuya asks Mikuru something and Mikuru tells her in a deadpan voice: "Google it."
When the SOS Brigade has a fire drill, Haruhi tells Churuya-san to stay behind and put out the fire, armed only with a water pistol. She fails to make it outside in time for Haruhi's roll call, so Haruhi declares Churuya dead, despite her peeking out of the classroom window.
Kyon telling Ashakura that thinking of him causes her to have a faster heartbeat and trouble sleeping because she's going to die.
The video game Churuya's Great Adventure, especially when Churuya-san suddenly starts flying through the air and shooting ships. It Makes Just As Much Sense In Context.
Kyon's reaction to being in bed with Churuya and Ashakura sleeping on either side of him: