When people get on, ask for their tickets.

When there's only one other person in the elevator, tap them on the shoulder and then pretend it wasn't you.

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?"

Drop a pen and wait until someone reaches to help pick it up, say "that's mine!"

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.

Ride naked.

Push the top floor button and announce that you tried to kill yourself yesterday but the other building wasn't high enough.