Are you on a creek, mountainside stream, lake what?
I've looked into it, it gets complicated, but depending on the amount of power you are looking to generate, you might be happy with a bicycle powered generator that converts your bike to a stationary generator, forget the name, but it clips to the axle and the tire spins the generator.
RV style solar panels can set you up with 12V for relatively short money.
All depends on how much juice you need.
Like they say at the speed shops...
How fast do you want to go?
How much do you want to spend?
I think this is pretty cool... a little long, but good information