Do you have any resistance bands or free weights ? If so, you can do upper body strength exercises while sitting in a chair. Work your upper body muscles while your ankle heals. An upper body strength workout can be quite challenging.
Also, if you are looking for more chair workouts, try YOUTUBE. Is your doctor letting you walk with crutches ? If so, use them. walking on crutches is surprizingly effective exercise. I knew people with broken legs who lost weight because they had to use crutches.
Mostly, do the best you can. If you can't exercise as vigorously as you want, don't beat yourself up. Give your ankle time to heal. As long as you are mindful of what you eat, you will not gain weight because you can't exercise. When it comes to weight loss or even weight gain, what matters most is what we eat. Good nutrition is what takes the weight off and keeps it off. Exercise is what keeps our bodies fit and healthy.