How do I get power to my fort?

Its summer, Ive been making a fort out of trash or junk people dump out here. I have the fort in the middle of a field, there's a pond next to the fort, its surrounded by trees and there isn't an outlet for like half a mile. i wanted to know if i could make something to generate power (like a windmill, an electric bike, solar panel) that wouldn't cost much and could keep the energy stored some how. I would like to keep the final price around $50. Thanks -Brandon