The Massage Prank
Step 1 Build a Shack or get a shack on top of a big steep hill.
Step 2 Name it something like: "Full Body Massage" (Flyers are optional).
Step 3 Rig it so that when they wait for their massage they have sit in a chair
Step 4 Wearing nothing but a towel and wait for their massage person to come.
Step 5 Put the chair so that the back of the chair facing the wall.
Step 6 About 20 seconds into their waiting time
Have the wall open up and both chair and person slide down the hill and into pool full of leeches. Would probably work better if chair was on a track, again optional.