Hello, Cornellchick!
I have to really take care of my knees. I am very keen on Callanetics. (Have been doing them off and on for twenty years and very consistently for the last three.) They build strength, increase flexibility and endurance and you have control over how you take care of yourself doing in every aspect of each exercise. To me, and for many others, Callanetics are a miracle.
I know many who swim because that's the easiest on their knees, or hips, or whatever they have to be very careful of.
I like things I can do at home or in my own neighborhood. Don't know if any of that would appeal to you, but thought I'd post it, in case it does.
Editing to add, due to the Brown's post that popped up just before mine, that I quite enjoy my rebounder, too!
All the best to you!