A corrugated plastic tube which, when whirled around at speed, produces a note.
Demonstrates standing waves, and how exciting a system more or less may result in differing resonances.
Extend the tube out all the way and whirl it around your head. If not much sound is produced, try holding it from the other end.
By speeding up or slowing down your rotation you can play different notes. About three different notes should be achievable, a fourth may be reached with great effort.
When played, the tube will produce a certain note that's dependent on it's length and density of corrugations. Increasing your speed will increase the pitch that the tube produces, but only in discrete steps, as this increase represents a higher-order standing wave.
Some more information can be found at the wikipedia article.
Demo room information