This is a prank I've always wanted to do in an elevator.
Get a lion dance troup to hide int there and whenever the door opens they beat the drums and cymbals and get the lion to jump at the person who's going in.
Grab one of those really loud blow horns and once the elevator stops so someone can get on right when the door starts opening press the blow horn button!
Take a violin into an elevator. when you stop on any random floor, start playing badly on purpose. play as loud as you can, and say 'This is my music session! get your own music session!'
Pretend to be stuck, and when the elevator stops, go up and hug the person who got the elevator, and say 'I thought i was stuck in there forever.' Keep preventing them from getting in it, and when it closes, just say 'oh well. your loss.'
Walk into the elevator. First push all the buttons. When somone gets on wait till somone asks you What floor? Tell them "All of them!" They'll either stare or say "Why?" if they stare say "what are you looking at? I want to go to All the Floors in the building" Or if they say Why? Reply Why not?
Take a water bottle into an empty elevator, and wait for the floor to stop. then, (if you're a guy) pretend to unzip your pants, and stand in a corner. Uscrew the water bottle, and start pouring it to look like you're trying to go to the bathroom. When someone looks over at you, just say 'a little privacy please?'