For fun and something different to do, consider this remote-control rubber band launcher by Instructables user "powellcubs". It uses a small servo to create the tension and let the band go, and the system is controlled via infra-red - so you can place it out of the way or in hiding to create some interesting surprises. Reading signals from household infra-red remote controls is quite easy, and the receiver can be found for less than a few dollars.
if you're looking for a small Arduino-compatible board to embed in a project, consider our LeoStick. It's the Arduino Leonardo-compatible board that's cheaper and smaller than the original:
Apart from being one of the smallest Arduino-compatibles on the market with USB, it also has an onboard RGB LED and piezo which can be used a knock sensor and various tune and sound effects. Plus you can add extra circuitry with the matching protostick! For more information and to order, click here.