Spiral Trip
If you're alone in an elevator, and you don't particulary feel like being nice, you could fart in it just before you leave it and the door closes, so that someone else can throw up, specially if you've had something really lousy for your previous meal.
Another more innovative thing you could do when you're alone in an elevator (a sizeable one, at that) is to move clockwise/anti-clockwise in circles while it is travelling up or down. It feels like you're moving on a spiral staircase. The feeling is really cool too :), and it gets you all dizzy once you're stationary.