It is perfect for kids still at school as well; it can be made by dads-and-lads together, or by competent older children alone, and it can be calibrated in Newtons and used to do home experiments to reinforce schoolwork (any UK readers with kids in Years 6 or 9, this will help them get ready for their SATs).
Step 1: Materials needed.
Step 2: Preparing the tubes.
Drill or melt holes in the tubes. You need two in the wider tube, both near one end, opposite each other, and four in the narrower tube, all at the same end, spaced every 90 degrees around the tube).
Smooth the edges of the holes to stop friction wearing through the rubber bands (you will not need to do this if you melt the holes).
Hold a nail in pliers, and heat it in a flame. A candle or lighter flame will do, but this is a case of "hotter is better", so I use a blowtorch or a bunsen burner. Use the hot nail to poke a hole in the plastic tube, being careful to avoid inhaling the noxious fumes.