Out of fun and interest, I built a Theremin-like instrument, being played entirely touchless with both hands. The idea was to use simple distance sensors with a digital output to do the audio synthesis on a computer. This allows for interesting and fun ways to tinker with the instrument, such as "auto-tune", different audio waveforms or multiple tones at the same time.
Please visit the project homepage for a demonstration, detailed instructions about how to build and use such an instrument on your own, and the general development of the project.