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Hmm... How about a bicycle air pump? They even have the ones with the check valve that pump air in both directions. Most are good to about 100 psi, and the foot-pedal operated ones seem to move a decent amount of air, so gang them together and connect them up to an air tool (grinder? screwdriver?) to drive a small generator. Kinda Rube Goldberg, but it might work.
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