I started at 350lbs and I feel that swimming is the best exercise to start out with. not only is it better on my joints but it's fun to me, too. I am out there swimming literally almost everyday because I love it, it doesn't feel like exercise but at the same time I really work out doing it. And it doesn't have to be expensive if you don't have a pool. One of those small above ground pools from walmart is more than enough room to workout in.
Now that I've lost 80 lbs I'm challenging myself to learn how to ride a bike this fall. But I would absolutely love one of those cycles you use while sitting.