Sitting on a saddle (on the floor) gives a YAFM if you have sufficient skill in riding rather than the standard “It’s not very comfortable…”

If you sit down and a cream pie is the top of whatever stack of items is beneath you, it produces a “Splat! You sit on a cream pie!” message and destroys the pie.

If you sit on a towel, give the YAFM “It’s probably not a good time for a picnic…”

When the hero sits in a corner while wearing a dunce cap, give a YAFM and exercise wisdom.

YAFMs (both regular and hallucinating) for sitting on the Vibrating Square. Something to do with a vibrating chair massage.

Occasionally, sitting next to a tree will cause an apple to fall on your head.

Sitting on gold should be comfortable if you are polymorphed into a dragon.

Sitting on the floor messes up things written in the dust.

Sitting as a Monk enables meditation, which can have useful effects.