If you want to work up a sweat while you're resting your ankle, try any of ab exercises that you do with your feet in the air, such as bicycle crunches, hip lifts, frogleg crunches, bicycling your feet in the air, and so on. When you no longer need to elevate your ankle, any seated exercises that don't put pressure on your foot will work, including seated shadow boxing and upper body weight work.
As for building your ankle back up, I agree with getting your doctor's help doing that correctly. A few sessions of physical therapy can show you the right exercises to do.
Be aware that you'll lose a little of your balance when your ankle gets flabby. You use those muscles to realign your body when standing and if they get flabby, you can fall or twist your ankle again. Be careful while you build them back up.
Have a speedy recovery but don't overdo it.