a few that I recall from my college days
1. If their in a smaller dorm, only a few floors with plenty of outdoor space, take all their stuff, and recreate their room outside, but you have to be their to see their reactions, or at least have a hidden camera to catch them, this goes without saying, only do this if the weather is good, and make sure someones keeping an eye on their stuff so nothing walks away
2. saran-wrapping toilets, need I say more? (just make sure you get a nice tight wrap so you can't see the wrinkles in it, but keep in mind, after this one, the war is really on, as this one gets really really ugly if successful)
3. duct taping the doorway, yes, good old duct tape, cover a good portion of their doorway with it, when they open their door in the morning it'll be to a wall of nice sticky duck tape, big pain in the ass to get it down and off your arms. Get creative, taping sheets of plastic up covered in other harmless sticky substances could be equally entertaining, just make sure you don't make them late for a class or anything.
theres more, but I'll have to share them later, off to work I go