Of course, there is always the more traditional elliptical-type drive found on ingo bikes (Once used on a Three Stooges movie). Peter Wagner makes them and calls them Whymcycles. Basically, they are scooters with the rear hub set off-center in a 24 inch (or larger) wheel. As the wheel goes around the rear end goes up and down. You approximate gearing by moving forward or rearward on the deck. They are quite fun, but they are not bikes, legally anyway. Gotta have a seat and pedals to be a bike. It would be pretty tough to maintain 20 mph on one, but 10-15 is doable and anything over about 4% grade will be a tough climb. Google Whymcycles for pictures or just go to Burning Man or any Kinetic Sculpture race on the West Coast and you will see some. He makes them from discarded bikes (a recycled non-cycle) and sells them pretty cheap. My family always thought of them as a means of low-impact running.