I recently started training a client's wife and she came in one morning and said, quite smugly, "It's too bad I can't do the same exercises as *Greg, seeing as how I can't do any high impact exercise." She conjured up a frown. "I guess I won't be able to to burpees or mountain climbers, like he does. What a bummer," she added with a disappointed sigh.
What she didn't expect was that she would indeed do both of those exercises without any impact at all (and that she would likely keep her mouth shut in the future, lest she give me anymore ideas).
With the right tools, you can take the impact out of a number of cardio exercises without losing the intensity, and my favorite tools for this are Gliding Discs. I've highlighted a variety of exercises with the discs (like sliding lunges and pushups), but mountain cilmbers are one of my favorites.
Not only do you engage both the quads and the core (and, boy, will your legs burn, as my client found out), pumping your legs as fast as you can will get your heart rate up, making this a great total body exercise.
If you don't have Gliding Discs, you can try using paper plates or even towels, if you have hardwood floors.
Do it right: Begin in a plank position, toes on the Gliding Discs and alternate sliding each knee in towards your chest, 'running' as fast as you can. Bring the knees in as far as you can while keeping the hips down and the core engaged. Repeat for 1 or more sets at 30-60 seconds.