Here's one that my roommate and myself did to a residence buddy. One morning (early) we taped together a bunch of sheets of newspaper to cover the victims doorframe. Then taped this big sheet over the doorframe which left a gap of about two or three inches between the sheet and the door. Then we filled the gap with paper balls right to the top of the doorway. When he opened the door he was showered with a barrage of paper balls (makes a nice mess too!) Of course, the door has to swing in for this to work!
My roommate was (and is) rather inventive and can be quite nasty. He buttered all of the toilet seats in our wing of the residence (fortunately told me first). He also buttered doorknobs at one point. We wrapped celophane over the toilet bowl then replaced the seat: this one can be really messy!
Try this: hang a shower curtain out a window. When the person below reaches out and pulls it in, pour a bucket of water onto the shower curtain. Listen to hear the results. Requires a nosy neighbor below you.
Six friends of mine and myself tried a less complicated version of the classic dismantling of a car and putting it back together somewhere strange. We lifted a friends car that was parked between two other cars and turned it so that the front and back end were facing the neighboring cars. This posed quite a problem for our friend when he decided to go home. Requires either a small car or a lot of very strong people! :-) I take no responsibility for any back injuries that result from this. Of course I take no responsibility for any of my actions. :-)
There are, of course, some fairly standard pranks that are pulled in residence. Typically, people are shafted on their birthday which is therefore a hazardous date to reveal. Total demolition of a room is quite common, but lacks any real humor. A common shaft is to remove everything from the victims room and set it up somewhere else exactly as it was. The best examples I saw of this were: a room moved to the roof of a science building, a room moved to the front courtyard of the residence (really funny when it started to snow!) and a room moved to the dining hall.
When I was younger, I had a practical joke genius for a working companion. We both worked in the same computer store for a while. He left and became manager of another store. I remember receiving an envelope with his firms return address on it. Inside was a very silly brochure. I said aloud "There has to be something else in this envelope". So I looked and of course there was a sheet of paper. It read "No there is nothing else in this envelope!" I could never get him back for anything that he pulled but he was an inspiration. The last practical joke that I will relate was one that he taught me and it requires a bit of time to prepare. First you need: iodine cristals and some amonium hydroxide. Mix the two together and a brown sludge will form. Drain off the excess liquid and let the sludge dry. The result? Snap powder, a pressure sensitive explosive. Just sprinkle this on the floor and watch people's reactions. Its quite amusing.