Designed for children ages 10 and up, the Triple Action Solar Car from OWI is a great introduction not only to the principles of solar power, but also basic electronics, switches and gears.
Triple Action Solar CarThe kit, available from Amazon, includes just about everything you need to build a very basic toy car, about a foot long, that can run off of either solar or battery power, and has a simple mechanism for switching gears from high speed to low speed. I say "just about" because is doesn't include a AA battery (which isn't required for the solar option), and assembly requires some use of tools.
The instructions are very clear, and assembly took about 45 minutes. Most of it snaps together. For the more intensive assembly, the kit recommends using a hammer and exacto knife, but I was able to do everything just fine with the handle of a butter knife and my teeth. There is some stripping of small wires and threading of those wires through some pretty tiny holes, so bigger hands and shorter patience may become frustrated. But generally speaking, it's a pretty easy assembly.
I had two small beefs with the kit. First, even though it claims you can assemble it in a couple of different configurations, you pretty much need to decide which configuration you want before you build anything. Many of the pieces are stuck together using heavy-duty double-sided tape, so it's not exactly easy to pull it apart to reconfigure it. My second beef was that, even though the box shows a shiny silver body, the shell is actually clear, and if you want to paint it, you need to do that first. So if you want anything but clear, you'll need to wait while the paint dries, so the time from box opening to finished car may be longer than expected.
But once it's done, the car is a great introduction to solar power and basic switching. The car can be run either through a solar cell about the size of a playing card, or a AA battery. Bring the car outside, switch it to solar power, tilt the solar cell toward the sun, and off you go. The car goes in whatever direction you point it, but as the clouds come in or you block the sunlight, that car stops accordingly. My six-year-old, well below recommended age, still had fun following the car around, controlling it with a well-placed shadow. The battery option takes all the fun out of it, as all you end up with is a car that goes straight until you pick it up and turn it off.
The instructions do come with an explanation of how solar power works, but unless you're really interested in the science of it all, it quickly induces glassy-eyed stares.
The end result is that I learned something about wiring and switching and my interest is piqued in solar technology. I only wish the car was more easily disassembled, as I'm curious to see how the solar cell would power other AA battery devices. Overall a fun weekend afternoon project...if it's sunny!
You can get the kit here.