That has to be soooooooooooooo annoying. I would strongly recommend that you do nothing until your physio tells you you can. Even abs and back work involves the muscles of the lower leg and would hurt like h**l if you tried them.
If you did upper body work it would have to be bicep/tricep work. Any spinal twists etc also use the legs as stabilisers or support. Try one sitting down, twist TOWARDS you bad foot and see what you do with your good one! You use it as pressure to sustain the twist!
You could just about do crunches with your bad foot raised, but getting into and out of position would be iffy!
I know this sounds extreme but you did say you had damaged the ligaments. It is these that will sustain more damage if you start to work without a physio's direction. I'm off to an instructors site and I will ask the resident genius PT there if she has any suggestions.