Hi Amy,
I agree that swimming would probably be the most gentle exercise. Ha, I had a horrible experience with swimming though. I also have really bad lower back issues. I have what's called sacroiliitis. BRUTAL! Even doing a light swim made my back beg for mercy! I think it was the way my back was arching. Holy smokers, I'll never swim again! But give it a try and see how you do. Maybe you'll have better results!
I have my mountain bike set up on a special stand so that I can use it indoors as well as outside. It works well for me when my butt isn't too sore from my Fibro. It's one issue after another!