I've got lots of joint pain, and I pretty much can't do anything impact at all. I really love the arc trainer at my gym, which is very similar to an elliptical machine. I can really get the heart going and fly on that thing with no pain whatsoever. I also have a kundalini yoga workout that's got some pretty intense joint friendly cardio, but there is a little bit of jumping so I have to modify it. Kundalini is very free-form though, so it's easy to make modifications and my total lack of coordination isn't a problem
. I used to do step aerobics, they're good in a limited space and no impact, but I don't know if it would bother your knees or not. If the height of the step is a problem, just do it on flat ground.