Bust my knee in 2012
Has the physio given you excersizes (if not why not?) do those (edit: as per above are good)
Swimming can definitely be good but you have tomprotect your knee so little kicking flex from knee – either straight leg or as per above a board you hood between your thighs
Weights and upper body excersizes can add tone and help with cardio
Press ups, sit ups etc
If you can take the stairs not the lift, often going up is better than down discomfort wise
IME concept2 rower was not good when knee hadn’t properly recovered
If knee bothers you after excersize ice it (buy a reusable freezer pack) and also listen to your body and don’t do too much of whatever is making it hurt !