Using Radio Shack's Snap Circuits kit and a common 555 timer IC, Instructables.com member KRA5H designed this simple optical theremin to give you an idea of what playing a gesture controlled instrument feels like (or doesn't). You'll have to custom-build the 555 chip piece, but don't worry, he tells you exactly how to do that too.
Some purists will point out that an optical theremin isn't a true theremin, and technically they're exactly right. Optical theremins vary their pitch by the amount of light hitting an optical sensor, whereas traditional analog theremins used the difference between 2 RF oscillators and capacitance to both sense the player and produce a tone. However, not everyone can afford a $200 instrument or has the technical moxy to build one from scratch, so personally I'm happy when such a simple project like this comes along that can introduce folks to the idea of a theremin.
If you're ready to try building it yourself, head on over to Instructables.com for all the details.
Read: Build an Optical Theremin--Improve Your Snap Circuits by Adding a 555 Timer Integrated Circuit II (Instructables.com)