Have a cool mechanical project or tool that uses a pulley system as the main means of working? What are some ways to get the pulleys moving? There's multiple possibilities that are easy on the wallet, including things like heat-shrink tubing, shoelaces and urethane belts. They all make great pulley belts. A good pulley belt material is one that will have a lot of surface contact with the pulley.
For heatshrink tubing, first put a long piece of it around your pulleys to see how much you'll need. Overlap the ends by about 1 inch (2 cm). Cut it so that it is 1 inch (2 cm) too long. Remove it from the pulleys. Get a heat source such as a heat gun, a stove element, a candle flame, ... Pinch one end of the belt and push it into the other end. The amount you should push in is the amount that you cut it too long by (1 inch or 2 cm.) Apply heat to this pushed in area; but don't melt it, just shrink it. And you're done.