At Uni I was in Halls of residence for the year I was there. It was a group of rooms, 18 to a square with communal bathrooms in the centre and a kitchen at one end.
The doors would spring open if you kicked them just by the lock, thgis led to numerous pranks,
1) The usual 3 o'clock in the morning, burst in and dump said occupant in a freezing cold bath
2) unscrew the hinges so when occupant came home and opened the door with the key the whole door would fall into the room.
3) and this was the best, one lad had to go home for a fortnight due to a family thing. we broke into his room, now in each room you were given one of those 2' x 3' long shagpile rugs. We watered and grass seeded it and grew a lovely 'lawn' in his room, the day he was due back, we borrowed a student uni mini bus and went up Snowden(I was at Bangor).We nicked a sheep off the mountain and stuck it in his room. When he came back he opened his room to find the sheep happily munching on his new lawn!

( We took the sheep back and it wasn't harmed in the making of this prank!)
Then there were the times at work, we used to get stuff delieverd in big boxes filled with those polystyrene S shapes. One lad made the fatal mistake of leaving his car keys lying aroudn so we opened a window, locked the car and then filled it to the brim with the stuff.
One played on me when I worked at the Britannia, By the kitchens there is a big extractor fan that blows the hot air out from the kitchen. I worked out that in winter if you parked under it your windscreen didn't get frosted up for when you finished work so I sort of made this my 'space' I also used the same spot in Summer.... Thats when the sods used 3 Catering sized rolls of Cling Film to completely wrap my car up. What they hadn't realised was that the heat from the extractor actually turned it into shrink wrap! It took me about an hour with a sharp knife just to get in the car!
I've got some more but I'll leave them for later