For those whose knees or hips may make exercise really challenging, I am now a big fan of the Wii dance games, Just Dance and Dancing with the Stars. You can "dance" sitting down part or all of the time if you need to, but still get the heart rate up and sweat a bit. They both have a variety of music from several decades, so there is a good chance you'll find some music that makes it impossible NOT to dance. I've played both with 2 athletic types, and they both thought the games were fun and challenging. My favorite is Just Dance. It felt great to feel good exercising again. (BTW, when I successfully lost weight about 15 years ago, I started by "just dancing." I think I started with 10 minutes and just kept adding time before feeling ready -- and inspired -- to move to videos, then a women's gym.)
I have also been working with a physical therapist for several months. He's generously added a little cardio (arm cycling) to my sessions to support my weight loss. I hope to get enough range of motion back in my knees to be able to use a recumbent bike in the next few months.