I created a new demo today using the a creation out of the build your own vehicle area and the circuits demo. After taking a vehicle someone else had made and make some structural reinforcements, I took some rubber bands and secured a electric motor and fan blade to it. The power leads from the motor were attached to a small dynamo. As I held up the wires, kids would come up and crank the dynamo creating enough power to drive the fan blade to push the vehicle over the table. It worked pretty well and I think the kids enjoyed trying to move the car back and forth over the table.