Mousetrap cars are an old standby for science projects in grade school. The idea is to use the spring mechanism of a standard mousetrap to power a homemade race car. You wind up the car by setting the trap. The trap bar is tied to a string wound around the axel and when it is sprung, the spinning string around the axle forces the car forward. My son and a friend made mousetrap cars based on the best instructions we could find, which were on Instructables.
Cheap mousetrap, old CDs for wheels, covered in rings from a balloon for tire treads (increased friction), ball point pens for axles, and styrofoam chassis. We use a hot wire cutter for sculpting the foam.
The cars work okay. Don't expect a long ride. They are a good challenge for engineers. Have to figure placement of parts out. How to maximize the drive. Keep wheels parallel, etc.