At Maker Faire, I had a chance to make TapeTricity Cards with a few hundred people.
TapeTricity is quite possibly the simplest way to introduce electricity to people. THere were kids as young as three or four who made their own working circuits (the younger ones got some parental help). This project was fun, because people got a chance to make a drawing, which they then picked out some places for LEDs. Next, they built a simple circuit out of tape, batteries and LEDs. It is a fairly quick project, and costs about a quarter per person.
There are photos of this project.