If you're going to have a homemade electric car you'll need a way to charge those batteries. How about your own "water wheel". I found one via Make and the homepage for it has many designs and documents the process of making it. The one I find the most fascinating is the Pressure Wheel that manages to increase the water pressure to generate electricity. All you need is a stream and away you go.
I really like these DIY solutions with instructions. Even if you don't have the opportunity or resources to make them work for you they are always inspiring and a lot of fun.