Generate up to ninth harmonic of a fundamental sine wave, with controllable phases and amplitudes. Provides both audio and oscilloscope outputs. A good demo is to construct a square wave component by component--you can then compare the scope and audio output with a "real" square wave. Try asking the class if they can hear the difference at that point if the phases are changed--your ear is very sensitive to Fourier component amplitudes, hardly at all to phases.
Caution: it takes some playing around to get the phases right. This is best done before class. If you turn the thing off and on again, all the phases get scrambled. So, once the phases are right, leave it turned on--use an extension cord if necessary.