Handful of shaving cream, feather to the face or hand in warm water are the old standbys. You could also dust their hair with baby powder so when they wake up they look like they're 90. If it's a sleep over, swap clothes between bags so the morning is utter chaos.
Use a spray bottle with warm water and spray the back of their pants so they think they peed themself and if theyre in a sleeping bag and easy to carry Drag them in another room or outside if ur evil. Careful though I did this to my friend and she ignored me for two weeks cuz of it. :D
make a little bomb... waters+baking soda... just a small bottle aways from them the loud bang will definitely scare them.. Was gonna say make a MRE bomb. my brother who is in the Marines taught me how to make a MRE bomb haha there fun and not dangerous at all.. or you can slip a Baking soda bomb under his mattress:)