Best Prank I've ever seen was :
This guy goes bungee jumping with his friends, and right when he jumps off his friends start shouting "NO WAIT!!!" and then throw a rope down behind him. Since I doubt bungee jumping is an option here are some classics.
Couch coffin - when someone sleeps on the couch you take another couch and put it upside down on the first one so when he wakes up he's trapped.
Shrink wrap to a table, dexter style for added fun.
Duct taped to a wall or post is always good.
Human wedgie pinata - When a friend passes out, you tie a rope to his underoos and toss it over a tree branch. You then have someone else hoist him up off the ground by his underwear and proceed to beat him with foam bats... or real ones, choice is yours.
Sharpie on the face is a classic.
Tuck a pair of the BIGGEST FREAKING PANTIES YOU CAN FIND, nothing smaller than 4xl will do the job, and stuff them in your friends pocket. Bonus points if he has a GF.
Freeze mentos into ice cubes, and make soda time bombs. (works best with diet soda)