Well, here is a list of some of my favorite pranks that I have experienced for myself:
1. Put Thousands of Stickers Everywhere In Their House, Car, or Bedroom: I did this to my friend about 3 weeks ago and he is still finding my Hello Kitty stickers in random places.
2. Put Baby Powder or Flour in Their Hair Dryer: Okay, now this is a very common one, but it still gets most people and is really hilarious.
3.Mess with your friend's computer by using the Print Screen button to copy the desktop and save the picture. Then set the picture as a desktop background. Hide all the shortcut or program icons and watch as your friend is trying to click on them and wonder why they can't open any windows: I never done this one before, but instead I had a friend do it to me. I literally spent almost a full hour trying to figure out what the hell was wrong with my computer.
4.Put powdered koolaid in their showerhead and then wait for them to be drenched with red/blue/ or whatever color you choose koolaid. It's fucking hilarious when your friend is screaming out of tje shower and wondering what the hell just came out of their plumbing system. If you know someone that likes watching horror movies, I suggest using red koolaid.