The Hachiyou Shou OAVs are a pure comedy. Highlights include:
Yorihisa and Takamichi failing to start a conversation. Sure, the topic was somewhat sensitive (they suspected each other of stealing), but performing the good old "no, please, you first" routine when you need to clarify something is generally not a good idea.
Inori's One Side of the Story-caused conversation with Eisen, which fails to include its main topic (Inori's stolen doll). Keep in mind that Eisen has absolutely no idea what it was all about because he did not take the doll and likely didn't know it even existed. One can only imagine just how he could possibly interpret Inori's behavior, with sudden talks about being lonely and such.
Yasuaki's Stealth Hi/Bye trait, and, as a consequence — him usually overhearing only parts of what other characters are saying. Directly leads into Tenma producing a string of stupid puns to get out of the misunderstanding without revealing what he actually said.
Akane meeting the Yasuaki mouse in the manga and Hachiyou Shou episode 5. It's technically more "cute" than "funny", but still fits.
Much of Hachiyou Shou episode 18; specifically, Yorihisa's overreaction at having to play the role of a thief, and Eisen unwittingly suggesting replacing Akane in her role with a crossdressed Hachiyou which got him Disguised in Drag.
Any scene in the stage plays that involves the Onryou trio. This gag reaches its climax near the end of Oborosoushi, where (as an alternative to complete depowering and disappearance, mind you) they get turned into toy-like "shikigami", so cute that the main cast keeps grabbing and adoring them much to their protests. Heck, this scene made Yasuaki smile!