This isn’t really a project because you don’t actually build anything. But using a solar battery charger requires strategy and scheduling, and it gives a good lesson on solar power and PV panels. This is probably one of the easiest ways to start using solar power. Chapter 9 explains what you need to know about getting the most bang for your solar beam.
An off-grid reading light is a great project, and you can get as fancy as you want. Start with a simple, self-contained camping light comprised of a small PV panel and a little pod containing a rechargeable battery, a light sensor, a switch, and a few LEDs. Then hang this light over your favorite reading chair (you may need to monkey around to get the light to shine in just the right spot), and you can read every night for hours without plugging in to the grid. Get the lowdown on these lights in Chapter 9.
For those of you who are more ambitious when it comes to electronics, you can design your own circuit (they can be very easy or very clever and efficient). You can design your own optics, which is a real gas if you know a little about the subject. You can get cheap optics parts from Edmund Scientific.