The lovely electrongirl is right - see your doctor, heres some ideas to run past him (perhaps not the silly ones):
Gym - pretty much anything done sitting you can do so most of the weights and a suprising number of machines.
Rowing Machines - it depends on their layout
Pushups on the knees (depending on where the plaster is)
Walking - yup I know you only have one leg but if you can move its called walking. If your on crutches think of the upper body workout its giving you. Take it easy though.
A lot of pilates can be done - just youtube a video and pick the bits
For the adventurous :
Canoeing - wrapup the leg in gladwrap and plastic bags and join in at the beach (only works if theres no waves).
Bike riding Tandem style (its important that your at the back)
Sky diving: actually a suprisingly good cardiac workout and no legs required
Seriously though if you did it 10 weeks ago the end is in sight. Perhaps work on the upper body, weights, abs etc and ease back into the cardio slowly. Swimming would be a good place to start after you get your boot off.