Last year some girls put the school's millenium statue (an unusual little thing which cost £20,000) on E-bay....and the headmistress' house (which is in the school grounds). Bit dubious about the legality of that though...
Another quite funny one was; put poppers (no not the drugs, the little cap things that go pop) under the toilet seats so that when people sit down they go off.
The most common one in my school is to use the gym pants though. If your school has regulation gym pants then there is so much you can do with them. In case you don't know what i'm on about, gym pants are giant umm.. well pants really, that girls (i don't know if there is a boys equivilant) wear with a t-shirt as an entire PE outfit, but they are hideous (my schools aregiant and maroon, hence the nick-name marooners). String them up to make very fetching banners, put them in local landmarks, tie them to other (or even better rival) school's railings...the list goes on!
I would reitterate the don't do any damage thing though, and also don't throw eggs. A nearby school threw eggs at some of the younger girls who go to my school, the egg shell cut one girl's face and eye -she ended up in hospital and still has a scar.