Just wondering if anyone here has any ideas for a safe

and relatively legal senior prank. it won't be til next year but it has to be awesome so we're planning ahead =). Building's open [to us anyways] at night so we can basically do whatever we want, but were a small school <160 students and only 25 in my class. As far as i know none of the classes have done pranks and we want to do one. at the moment, we have plans for bird seed all over the parking lot, TP'ing something, possibly 'decorating' rooms with tin-foil, newspaper etc. Water-filled cups might work. We were thinking about taking over the PA but there's an elementary at the other end of the school and we haven't decided whether we want to mess with them or not [odds are not]
*note: we are going to clean it up and most likely we're not going to get in trouble, but just to be safe we'd like to keep it legal. also seran wrapping and egging cars destroys paint so thats off limits.*
=D