Michael Una makes and sells these cool hand-held “Beep-It” synthesizers:

Beep-it Deluxe is a handmade optical theremin, housed in a handsome laser-cut bamboo case and featuring a big chrome button and 2nd oscillator for enhanced modulation.Beep-it Deluxe emits a square-wave tone reminiscent of 8-bit video game music. A light sensor determines the overall pitch of the instrument, while a knob on the side can be adjusted to induce a warble into the tone for all of your sci-fi and rhythmic needs.

Check the vid for action sights and sounds. Shown above is the Beep-It Deluxe; Michael also offers a basic model with the same functionality but a less flashy enclosure.