Stand by the door and hum the first seven notes of "it's a small world after all" over and over again.
when someone comes on the elevator ask them if they need toilet paper
dont let anyone leave till they give you a tip(MUHNEHZ)
whenever the elevator goes up, giggle, and when it goes down, scream like you're on a roller coaster
brush your teeth on the elevator when lots of people are there
stick a rolled up towel un the door, one half inside one half out, and have someone hold the one end while you hold the other
play annoying ringtones
I have lots more.... :D