Practical Joke Items
Get a piece of plastic as long as the victims bed and a little over twice as wide. Remove his mattress from his bed frame and line the frame with the plastic. Tape the plastic to the bed if necessary. Fill with water (a hose connected to the tub is helpful). Fold the rest of the plastic over the water, and make the bed. (Done to my roommate by mutual friend.)
If your victim has a roommate, switch all their possesions. Or, turn all the posters upside down and hang the furniture from the ceiling. (Both done to me.)
If your victim _is_ your roommate, switch the material in your waste baskets and pull his bed away from the wall a quarter of an inch every day.
Wait till your victim is away for a weekend. Dip everything plastic (pens, phones, etc.) in liquid nitrogen, break it, and stick it together again. Everything plastic will break when he picks it up.
Polish the floor and stick teflon to the legs of select items of furniture. (The polished floor is a bit of a giveaway, but teflon isn't dramatically slick if there's much grime.)