Water Aerobics would definitely be the best low-/no- impact but it's not always convenient. The recumbant bike is the next best thing but I agree with PhotogGal that bikes vary and some are easier to use than others. Do you belong to a gym that has personal trainers available? If so, have a few sessions with one to get advice on things you can do and always make sure you let them know when there is any pain, not the "Oh my muscles are tired and I want to stop pain."
I had a knee replacement last year due to my arthritis and I used the recumbant during physical therapy. Now, I am on the treadmill walking and some running, using the elliptical as well as the recumbant. The knee replacement was the best thing that happened to me this past year!
Of course, my meds have helped a lot too! They've gotten my pain in check so I take advantage of these pain-free days to be healthier.
If you ever want to chat or be buddies, let me know. I'd be happy chat!