I would second Lenore with cycling. From my own experience, regular cycling really strengthens your lower parts. I also like jogging, but only outside. Stationary bike is good for bad weather days - I usually put on a mountain movie of some sort and cycle through
When I had a painful knee injury a couple of years back, I discovered, that biking puts much less pressure on your knees than jogging.
On a funny side, I bumped into an old friend once (she hasn't seen me for years) and I was complimented: "You have thighs like Marit Bjoergen!"
Wasn't sure if I should laugh or cry...