Lets see... some friend made Couchhenge by taking all of the lounge furniture out of all of the dorm's lounges, then arranging them in the classical circle-henge-thingie in the middle of the dorm's open space...

It was huge and suddenly cropped up overnight... it rawked....

Also tormenting people with Lobo (Giant stuffed bear from a circus... easily 4 feet tall and HUGE) was fun...

Also,not much of a prank per se, but whe got a projector set up and once played Super Smash Bros. against the side of the building... the screen was at least 30' across...