When there's only one other person in the elevator, tap them on the shoulder and then pretend it wasn't you.
Push the buttons and pretend they give you a shock. Smile, and go back for more.
Ask if you can push the button for other people, but push the wrong ones.
Hold the doors open and say you're waiting for your friend. After a while, let the doors close and say, "Hi Mike. How's your day been?"
When the doors close, menacingly announce that "It's going to be a bumpy ride."
Constantly bounce a tennis ball.
Drop a pen and wait until someone reaches to help pick it up, say "that's mine!"
Stand in the corner reading a telephone book, laughing.
Take pictures of everyone in the elevator.
Turn off the lights in the elevator to "conserve energy."
Leave a box in the corner, and when someone gets on ask them if they hear something ticking.
When the doors close, announce to the others, "It's okay. Don't panic, they open up again."
Push your floor button with your nose.
Stand alone, and when the doors open tell people trying to get on that the car is full and that they should wait for the next one.
Swat at flies that don't exist.
Push the top floor button and announce that you tried to kill yourself yesterday but the other building wasn't high enough.
Talk to people about the "golden age of elevators in the 50's."
Stand really close to someone, sniffing them occasionally.
Ask, "Did you feel that? I felt a rumble."