Simple one really, but I thought may be good for the viewers;
A handpowered powersupply for recharging a mobile device (~5v) using a (stepper?) motor as a generator. Would make for good talk about filtering, regulation and recification maybe even dc-dc concepts. I actually got given a similar device a few weeks ago to look at and was shocked as it was literally a small hand wound motor with a single diode, limiting resistor and a usb connector. Absolutly terrible.