how about some chair yoga DVD's? I can get my Heart rate up just by sitting down, moving my arms around, and leg lifts etc. I'm sure there are many Chair DVD's you could combine with short walks and progress very quickly
edit: here are some. I'd probably start with the 1st (even though it says for seniors, it has good reviews by young people too!)
http://www.amazon.com/Stronger-Senio...1&sr=8-1-spell
http://www.amazon.com/Chair-Aerobics...1&sr=8-3-spell
How about water aerobics? I do a class once a week. I started out in the shallow classes, and moved to the deeper classes. I love the deep water because you can go at your own pace and no one cares. There are all types of people, ranging from very fit young athletes to very overweight people, to seniors etc.