Rattlesnake Egg Envelope
This is an oldy but good. Fake Rattlesnake Eggs. The fake rattlesnake eggs has rubber bands attached at either end, and each rubber band is attached to a steel washer in the middle. You wind up (or load) the washer, making sure it doesn't unwind as you slide the whole assembly into the envelope. Then fold over the envelope tab. When the victim open it up, it unwinds and sounds like a rattlesnake.