I had two ideas, water rockets or a dust trap, and am asking for votes on which I should go for. Both are described on my profile page, so far the water rockets is winning!
NPL produces a kit which allows you to shoot water bottles high into the air, learning about forces and aerodynamics and having a lot of fun while you do it! £500 would buy me 7 packs that I would like to send off to schools in poorer countries.
Check out my profile page for some pretty cool pics of water rockets in action.