I've spent the last two weeks making an Arduino-based "air" MIDI controller. Kind of like a theremin but with cheap sonar modules (distance sensors like they use on toy cars).

It's incredibly fun to play. It's not accurate enough to control a pitch with precision but for a controller, to drive filters, modulators, etc. it's perfect.

Also, my kids love to play with it -- not in a "musical" way but to control the music while they dance. It's a way of using it I hadn't anticipated and it's super nice to see them do it.