Good (friendly) booby trap ideas?

There's the packet of kool-aid in the shower head trick ("sulfuric acid"), the dummy made under his covers in his bed - you could put a fake "bomb" on it, the rubber snake in the underwear drawer, vaseline on the toilet seat (you can't see it until you sit on it). Those are all tried and true booby traps. Also after he goes to sleep build a wall of empty soda cans against his door so that when he opens it in the morning the wall comes down with a crash.