Volatic pile, or how to make a penny battery
You'll need pennies, cardboard, electrical tape and either vinegar, lemon juice or salt water. I'd suggest galvanized(zinc) washers rather than sanding the pennies. You won't get super electricity from these but you can get enough to power small devices or make small flashlights with decent bright LED bulbs(adding a tiny switch shouldn't be too hard). Hard to say how long the vinegar will really last though, so they may not be something you can just store away. Thin cardboard works better than paper and wrapping the battery in electrical tape should keep them wet longer.
More pennies means more voltage, while more batteries linked in sequence means more amps.
This is a battery pack I made with 22 pennies- two 11-penny batteries in sequence