The Eames Solar Do-Nothing Machine
In 1957, Charles and Ray Eames designed the Solar Do-Nothing Machine, a set of kinetic toys powered by the sun. It was created for an Alcoa advertising program intended to show the virtues of aluminum as a material, by creating unusual things from the metal.
The Do-Nothing Machine was a relatively early use of solar power, in a time when 8% efficient solar cells were cutting-edge, and wimpy 2% efficient cells were in commercial use. Given the limited power available to them, the designers decided to make a toy.
Is it sunny where you are? Perhaps you can convert some of that sun into fun:
Interesting trivia note: The Eameses filmed their contraption in action, but never released a film on it. Nearly forty years later, their grandson Eames Demetrios discovered the footage and made this film. You can read more about the contraption from The Kid Should See This, which also has links to more awesome Eames projects.
See also: Inside the Eames House; Powers of Ten; The Rise and Fall of Rome in 5 Minutes; and The Power of Simple Math.
(Via The Kid Should See This, which you really should be visiting every day.)