Post things that would be entertaining to do in an elevator.
Press every button but the floor the person requests and continue to do so.
Use chalk to draw a square right in the middle of the elevator. Stand in the square. Warn everyone that this is YOUR spot, and hiss at all who get within a foot of you.
Bring a boom-box and attempt to break dance.
Stand silent and motionless in the corner, facing the wall, without getting off.
Stare, grinning, at another passenger for a while, and then announce: "I've got new socks on!".
Bet the other passengers you can fit a quarter in your nose.
Walk on with a cooler that says human head on the side.
Say "I wonder what all these do" and push the red buttons.
Announce in a demonic voice: "I must find a more suitable host body".
Or better yet, cover yourself in a plastic suit and go with a friend then have him say "So I heard it was highly infectous... and fatal." Then say "Yeah, but wait... I think my plastic tore..."
sing the most random song when everything is quiet
When the door opens to a crowd of people, say "Sorry, this is reserved" and press the door closed button.
You get the point!