I have thought I would like to do this sometime too.

An easy way might be to take a junk Standard Pneumatic player action
and slice it vertically, leaving one to three valves and pneumatics
intact, then covering the end with polycarbonate.  ("Plexiglas"
acrylic is very brittle and hard to machine.)  Add a cross section of
a tracker bar and tubes and something that looks like a roll and light
the inside brightly.

If people can trigger a valve to move by putting their finger over a
hole, they will be more engaged and probably understand more quickly.
Alternately you could use a soft pedal pneumatic block and put a
transparent side on it.

Now if you could only make the color of the air change with the vacuum
level...

Joseph Osborne - Osborne Keyboard Restoration
Carlisle, Pennsylvania, USA