I was hiking along a trail one time, and I came up behind another hiker who had some apparatus hanging over his pack. I glanced at it and couldn't figure out what it was, so I asked, and the other hiker told me.
It was a Pelton Wheel. This was a very small-scale hydro turbine generator. This guy knew exactly where he was going to camp by a roaring stream, and he knew exactly where he was going to place the Pelton Wheel, and he had a good idea how many watts it was going to generate. I don't think he was going to run a toaster on it, but he had some ideas with lights and stuff.
The Pelton hiker and I parted ways at the next trail junction, so I never got to see it work. You think a crank or a PV panel are impractical, just go put up your own Pelton Wheel.