So I've been doing some research lately on radio waves, which will eventually turn into another project once I have enough *SPACE* for it. One of the circuits that always blows my mind is the simple crystal radio. The concept is that the antenna picks up enough energy to drive an earpiece and you can hear music with a ridiculously simple circuit.
Enough energy is collected by the antenna, then the diode only lets it flow to ground, causing a net current and the speaker makes sound.
What I was thinking: Why not use this feature of collecting energy to charge something? Obviously natural radio waves aren't enough to charge anything substantial, so a more powerful source is needed.
Why not equip a wristwatch with an antenna tuned to 2.4GHz, connect the ground plane to the metal base of the watch, and when you need to recharge it, just stick it in the microwave!
Disclaimer: This is most likely a really bad idea, it's just a musing.