Cardio without walking or running

swimming!! You can tuck a float between your leg. My local pool has a monthly membership option - even a daily guest pass, so you don't always have to shell out for a huge membership just to do a few laps. Bonus - it will fix any breath control issues you never knew you had

while biking also technically fits into that category, leg injuries often make that one difficult too. rowing may also be out, but also technically fits.

Yoga or some other form of body weight, isometric exercise (pilates?) may have some potential, with some modifications for standing poses. Your local library should have DVDs you can play around with for free.

Worst comes to worst, check out your local PBS station at the butt crack of dawn, they may have a chair aerobics broadcast. My grampa did that for a bit, but it's mostly for incredibly sedentary people attempting to maintain some limited range of motion.

Good luck, heal quickly!!

If you can go to a gym, rowing is a good option!

Depending on the injury, biking may be an option.

Definitely swimming. The elliptical and biking are also lower impact for knees and hips if that is where the injury is.