Twist open the shower head, poor in any color kool-aid-(best red) and then twist the head back on. At a restaurant, when your friend isn't looking open a ketchup packet, stick the straw in, then put it back in their drink. Use an unopened water bottle, and poke holes into it, when someone opens it, it will spray them in the face. Saran wrap the toilet seat... Wet the cardboard of the toilet paper, then place it in the bathroom.