I like where Youtube user DanieleMattei is going with his video Filter control via light blob tracking in Processing. Using OpenCV and Processing (what else!), he rigged up a way to control the parameters of his MIDI synthesizer by waving a flashlight around. It doesn’t look too complicated to set up, and might be a fun way to compose something as a group. With a web cam built into your laptop, you could set up a mobile sound station that anyone with a flashlight (or bright cellphone screen) could play.
Update: Daniele mentions that he developed this project with Massimiliano Schinco, as part of their studies at the ISIM garage. Massimiliano also has an updated video of the project. Great work, guys!