Peekaboo. There are dozens of good variations here. Tonight’s was Dante standing on my shoulders, facing me, while Laura walked in front of me from one side to the other. It cracked him up to have her walk behind him, disappearing from his field of vision only to appear on his other side.
Hit a pillow that he’s touching.
Various tickling games, including the ever-popular “face in the belly.”
Toss him up in the air.
Hold him at arm’s length while spinning around.
Make a high-pitched “eeeeeeeeeee” sound.
Hold down a rubber duck or other buoyant object in the bathtub, then let it spring back up to the surface.
Pour popcorn from a measuring cup into a tupperware container. (Yeah, his sense of humor is sometimes quite inscrutable to us.)
Do yourself anything that he would normally do. For instance, put a pacifier in your mouth.
A close corollary to this one: Do yourself something you’re about to do (or would usually do) to him. For instance, wipe your own face with a washcloth before wiping his.
Do crazy stuff with your face, like stick your tongue out while shaking your cheeks back and forth.