Build a rubber-band-powered car from things you already have around the house. A toy car with a cardboard body, thumbtack wheels and axles from a pen's ink tube can built almost for free after a rummage through your junk drawer. The resulting rubber-band-powered car may not be incredibly durable, but it will provide a lot of fun while it lasts.
Remove the ink tube from inside a pen. The best ink tubes to use are of a standard diameter and transparent. Blow the ink out into a paper cup to be thrown away. Carefully (without getting ink on anything) transfer the tube to the sink and allow a trickle of water to rinse through the tube and down the drain until the ink is gone. Run a pipe cleaner through the tube with dish soap to remove any remaining ink.
Measure and cut with a craft knife a 1 1/2-by-1-inch piece of cardboard (unbent) so the 1-inch corrugations are running side to side across what will become the body of the car. If these corrugations are not lined up properly, the car will not roll straight, since the axles will run through these corrugations. To better see corrugations, lay a crayon flat on the cardboard (sideways) and colour lightly until the lines appear. Using the craft knife, cut out a rectangular 1/2- by 1/4-inch hole from the back, centred between the sides. The wider dimension should run the long way down the strip.
Cut two axles from the pen tube with the craft knife, making each axle 1 1/4 inches long. Cut up the remaining tubing and glue it around the hole in the cardboard to reinforce it. Start by cutting the leftovers into two pieces. The first piece should be 1 inch, and then cut in half again to form 1/2-inch pieces. Cut the second piece in half and insert it (coated in glue) into the corrugations on either side of the back axle hole to reinforce the structure. Cut off and throw away any protruding material. Glue the two 1/2-inch pieces on the underside of the structure on either side of the window to finish framing it in tandem with the two pieces glued in place internally. Allow the glue to dry completely.
Insert the back axle through the cardboard corrugation holes on either side of the back axle hole. Make sure that the middle of the back axle is exposed in the hole and that the axle is lined up properly. Cut the rubber band at one location and make a small incision into the tube with the craft knife and pull the end of the rubber band through. Tie a knot in the end so it does not slip back through.
Straighten a paper clip using needle-nosed pliers. Bend the paper clip so that it resembles a staple with a centre section that is the width of the hole around the back axle. Bend the middle section slightly toward the axle in the centre. On both ends of this staple, form two little loops around the axle. The car should have the back axle exposed in the middle and a "staple" on the axle that can toggle back and forth. Tie the other end of the rubber band to the dent in the middle section of the staple.
Glue wheels to the ends of the axles. Use wide thumbtacks that are glued firmly in place. Apply a thin layer of rubber cement to the outer rims of the drawing pins. Start to wind the back wheels until there is resistance. Slowly continue, allowing the rubber band to wrap around the wheel until the resistance is enough to get the car going. Be careful not to wind the car too hard, or the back axle can rip out of the cardboard.
You could fill every corrugation (not containing an axle) with glue to add rigidity and strength for a more durable vehicle. You could also paint the surface of the car and wheels with a thin coating of fingernail polish. The first coating will help prevent the cardboard from getting soggy if it is completely dry when the second coating goes on.
Don't get ink on anything that is not to be stained. Be careful with sharp objects, glue and associated fumes. Always wear eye protection. Keep the finished car and all of its component parts away from small children.