Change pitch of organ pipes by changing sound velocity.
What it shows:
At a constant temperature, the speed of sound in a gas c, is dependant on the molecular weight of the medium
where γ, R, T are constants
Helium's low molecular weight results in a higher speed of sound than in air, and for a particular wavelength gives a higher frequency. Sulfur hexafluoride on the other hand, is a really heavy gas (five times heavier than air), resulting in a lower frequency. This can be demonstrated clearly by blowing the various gases through organ pipes.
How it works:
Use a wooden organ pipe (see a selection under Organ Pipes) to blow a note using three types of gas. Your lungs can supply nitrogen, and cylinders of sulfur hex and helium connected by rubber tubing for the others.
In addition to blowing through the organ pipe, you could do the old trick of breathing the helium and reciting Shakespeare. Sulfur hex might contain toxic impurities, but you can show its weight by creating the proverbial 'lead balloon'.