I would second the free weights, get some fairly light ones (they're cheap or maybe you know someone with a set who'll give or lend you a pair). Also, is good to have someone who knows show you for form. You do a few sets of 10 of 4 or 5 different types of exercises
you'd be surprised how these can get your heart rate up. and you'll notice a difference in your muscles fairly quickly.
also side bends are a great exercise. it involves standing, but really 3 to 4 minutes of side bends is a good amount.
you can do it this way, or your arms along the side of your body without weights
I know those leg injuries suck big time, just had a knee one myself. hang in there whatever you decide to do. Think of it as a hiccup.