How about if there was a ride-on toy which you stood on and jumped, and the drive always happened on the down-stroke, freewheeling until your rate of fall made the rate of motion imparted to the wheels exceed the rate at which they're currently rotating? The advantage to this approach is that it can operate quite efficiently at a wide range of inclines and speeds, plus it's a fun bounce-bounce-bounce motion, and it has the benefit of being just plain different.
(I was going to draw an analogy with diabolo whipping, but all the web pages on diabolo whipping get the physics all wrong. Ah well.)
The mechanism for one of these turns out to be quite simple. There's a single wheel on a regular fork in the front, and two wheels on a single axle at the back. The platform you stand on is hinged at the front and you stand towards the back, right in front of the rear wheels. There's a spring under the platform to keep it pressed against your feet as you jump (alternately you could strap your feet in, but that sounds neither safe nor fun, and would increase the amount of impact on your legs). There's a segment of a very large gear sticking up from the back of the platform you stand on, and it meshes with a gear which can freewheel which is attached via a chain to the rear axle. The gear segment can't directly hit the rear axle beacuse it would need to extend downwards to mesh at the top of the stroke, which would then make it dig into the ground at the bottom of the stroke.
For maximum stability and control the front fork should angle backwards chopper-style and have a large wheel on it. There should also be a hand brake which goes directly to the rear axle. (I have an otherwise very nice jogging stroller which for some unfathomable reason has the hand brake go to the front wheel, rendering it utterly worthless when going downhill.) Yes I'm aware that this would look completely ridiculous, but this is my fantasy vehicle, and I like my fantasy vehicles to look like the world's dorkiest harley.
A nice feature of this vehicle is that it's in principle quite practical. It can go uphill, (it's a simple exercise to show that the gearing ratio directly controls the maximum incline you can go up at), can achieve quite decent speeds, is simple to ride, and is fairly low impact. The downsides are that due to the lack of variable gears it's a little bit of work to drive fast (but quite good at getting your leg press to a really good weight) and it has a very long dead zone in the stroke, so it can't sprint like a bicycle. For overall efficiency it's unclear how it would compare to a bicycle, but on general principle I'm going to guess it's less efficient, but my guess is not horrendously so.