Yeah biking or walking are the best cardio things I know of other than swimming.
You could also try some body weight strength training stuff. There are a lot of exercises you can do laying on the floor, on all fours, etc that won't put weight on your ankle. They won't burn as many calories as a cardio workout but it will improve your fitness and can even make you slimmer and more toned since muscle takes up a lot less space than fat.
Diet is super important for losing weight too. Just burning off the calories from one candy bar requires walking or running 2-3 miles, so you can literally undo all of your calorie burning with one extra snack.
Good luck! I hope you find something that works for your situation.