I really like SP's cardio chair workout. It's only 11 minutes long but an effective 11 minutes long and you can do it a couple times a day. I have problems with my knees as well (due to a fall years ago) so I am careful with my choice of workouts and I'm also over 300 pounds. I really enjoy bike riding. I got a cruiser bike (it has a bigger seat
) and I bike 5 days a week on it.