Almost all power is generated by heating water into steam, which can still be done by burning almost anything. It won't be very efficient, but you can do it - so traveling to the nearest coal mine/plant may be an effective energy source.
The most easiest thing, so, would be a Sterling engine then. Something I have to read up again, I'd love to build one with my kids. After all, it was the machinery we did hand-drawn technical 2D drawings for back at university before we moved to CAD.
