Build a stationary bicycle that has an alternator in place of the 'resistance' on the flywheel. Hook the alternator up to a regulator circuit to make 12 volts. Get some solar panels and hook them to an appropriate sized fan. Pedal as hard as you can to generate power. This will undoubtedly make you sweat. Use the solar panel to help you in generating 12 volts by keeping you cool.
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LOL. Throw in an umbrella and a bowling ball, and you've got a good start for a Rube Goldberg machine.
The only hard part is making sure that the spray from the Mentos & Diet Coke component doesn't muck up the solar panel.
Maybe that's what the umbrella is for....
Harbor Freight carries them. You can switch between 12, 9, 6 and 3 volts easily. I power a 12v computer fan with one. They're about 12 bucks.
Step 1. Purchase a solar panel that outputs 12V. Step 2. Place solar panel in a sunny place. Step 3. There is no step 3! Seriously though, lots of stores now sell 12V solar equipment, even places like Wal-mart and Canadian Tire. How much it'll cost depends on how much current you need. The more current your devices draw, the bigger the panels, and the bigger the price.