Best gag I ever did was a little manila envelope with a picture of a rattlesnake and conspicuous bold letters "RATTLESNAKE EGGS"
Inside was a washer on a twisted rubber band, so when the envelope started to open, the washer would spin and make a flap-flap-flap noise real quick, and vibrate the envelope.
It's a good gag, You're already on guard from reading 'rattlesnake eggs', then when stuff starts making noise and moving around it'd flat rattle some folks. One guy I remember jumped so far back his cigarette fell out of his mouth.