Active after ankle surgery

I start physical therapy next week but I wanted to ask here. At the end of December last year I had surgery to remove a haglund's deformity mass from the back of my left ankle. The surgeon sliced 50% into my Achilles and removed part of a bone and so it has taken longer for me to heal. I am just now trying to wean myself out of my walking boot per surgeon's orders and walking without a cane. My question is this, what types of cardio exercise can I perform while sitting or standing still? I have very little muscle in my calf but I can stand and walk short distances in my boot. Thank you in advance!