One Pipe Organ

Different gasses have different densities, which affects the way sound travels through them. Low density gasses tend to produce higher-pitched sounds, as when speaking in a high-pitched voice after breathing helium. Likewise, high-density gasses produce low-pitched sounds.

My proposal is for an organ with only one pipe. Instead of relying on diffrent pipe lengths in order to produce different frequencies, the organ will use different gasses to produce different frequencies. By pumping hydrogen or helium through the organ pipe, a high tone will be heard, while dense gasses like radon or uranium hexaflouride will produce low tones. Nitrogen or carbon dioxide could provide middle tones. Through careful chemistry, and possibly by mixing various gasses for some of the intermediate tones, one should be able to produce a full range of notes.

All that remains is to get a pressurized tank of each gas, attach it to a set of solenoid valves controlled by a keyboard, and start playing.

Naturally, not all of the gasses will be safe (flammable, corrosive, radioactive, etc.), so it's probably advisable to wear a self-contained environmental suit and avoid open flames while playing. Also, only one note can be played at a time, but that's a small price to pay for such an elegant display of physics in action.

Another small drawback is that each note will tend to start at the same tone as the last one played--the residual gasses will have to be cleared out before the new tone is produced, so you'll end up with a sort of glissando effect, especally with rapid sequences of notes. This could be mitigated by mounting the pipe sideways, so the heavier gasses don't get trapped at the bottom, but lighter gasses can still escape as well.