Here are some photos of the Grave Jumper.  It turned out quite well.  The pop up was activated by a motion sensor when the TOTs got close to the front door.  One kid dove for the ground when he saw it ans several had to be called back to get candy by my wife.  The strains of "you go first" could be heard throughout the night.  You can find plans for building one at http://www.phantasmechanics.com/jumper.html .