[MattTuck]

Louis hasn't chimed in yet, but I'm sure he would tell you to get a Concept2 Model D rower.

Similar economics to cycling. Buy once, cry once. Then the marginal cost of each workout is essentially zero.

Yes, might be boring, but if you put on a movie or TV show, and/or audio books or music, it isn't too bad. Probably equal or better fitness value than cycling on a per minute basis, as it does a bit more for your overall body.

I'd agree with looking at yoga as another option.

If you want something outdoors, you could try roller skiing. But not sure that is going to be any safer than cycling, unless you find somewhere low traffic to do it. The benefit is, you're not going to cover as much ground as on a bike, so your loops will be smaller, and so perhaps you can find more favorable roads.

One last one that I'll throw out is a racquet sport like tennis, or squash or racqetball. All of these can be hard on your knees, tennis probably more so because of the court surface. BUT, all incredibly good workouts and cover both endurance, hand/eye and some strength.

Tennis courts can usually be found at schools or parks for free. If your goal is really to get off the road, these might be ones to consider.