During College, as we had no desire to do any of that study business we used to pulled numerous pranks involving the elevators in our dorms. The first one we did was to get one of our more gymnastically inclined friends to hang upside down from the elevator ceiling and act asleep. The rest of us would then hangout downstairs and watch people's reactions when the elevator opened with this tall dude sleeping like a bat. The girls that were waiting for the elevator decided they'd catch the next one. Gymnast-friend felt like crap after having all that blood run to his head for way too long but some booze soon fixed that.
Next we grabbed a wheelie bin put it right in front of the buttons and then I hopped in. When the people got in they thought it was a bit strange that there would be a wheelie bin there but whatever. They were a little creeped out when the bin asked them "Which floor?" and a hand reached out of it to press the button.
The final one worthy of mention involves an idea we stole off the internet. After seeing a video of the infamous disco elevator we knew we had to try it. So after purchasing the cheapest discoball and light set we could find and raiding nearby op shops for seventies jumpsuits we quickly set everything up in the elevator put "You Spin Me Right 'round" on and jived our way to the bottom floor. It worked perfectly and as the door opened the music faded in and four of us were in there dancing like crazy with the lights flashing everywhere. The people waiting decided to take the stairs.