To learn about aerodynamics at uni, we made rocket cars, tested them in a wind tunnel and set them off in the car park.
It was so much fun I thought the kids at home would enjoy it too. We put a stick of solid fuel rocket motor (basically a firework) at the back of each model car, attached them to a line of kite string and watched them shoot into the distance or explode. Luckily, my neighbour Alan has a degree in this sort of thing so we were in safe hands.
The rocket motors were about £3.50 each from a local model toy shop.