Solarbotics has a new Instructible up on how to build a Solar Powered Battle Symet.
This looks really fun as these BEAM robots spin a lot faster than their predecessors and are easy to make.
From the Instructible:
BEAM Solar Symets (a contraction from “Symmetrical Robots”) were first built by Mark Tilden many years ago as a way to provide mobile plant-life to the other creatures in his Robot Jurassic Park. Being quite simple by only using one solar-engine circuit, they’re quick and easy to build.
These versions use high-power Miller Solar Engine circuits, that make them spin much more madly than their pokey ancestors! If you are new to BEAM, it’s a design philosophy that incorporates minimalist electronics, and often, solar power.
Buy the Solarbotic bundle of parts or source your own.