Well, if your doctor has given you the all clear to do some cardiovascular exercise, why not start by walking with crutches. Walking with crutches is surprizingly good cardiovascular exercise as well as upper body core and strength work.
Also, you could do chair workout DVDs. If you do a search on amazon, you'll find a selection of chair workouts you could do. And if you have a set of hand weights or resistance bands, you can do upper body strength work while your lower body heals.
Ultimately, you really should start with the suggestions of your doctor. They know the extent of your condition better than we do.