I agree on the Deep Water Running if you have an access to a pool - I used to do that a long time ago and WOW, you could really feel it.
I have been doing Zumba at home for a couple months now. On my cardio days, I usually burn between 600-700 calories in 45 minutes, and on the Sculpt and Tone days, it's usually 500-550-ish in 45 minutes. Your mileage may vary, however. I have issues with low blood pressure, and it seems like I tend to burn more calories than your average bear when my heart rate gets high. I use a heart rate monitor to calculate this calorie burn.
I've found that since I started doing Zumba I am noticing the most change in my core (waist and hips) and my thighs. There's a lot of butt-shaking and hip-wiggling involved. I don't know if it's a "rest" for your knees, however. A lot of the moves have you on one leg, and there's a lot of jumping around on the cardio workouts. Might not be as bad as running, but I would do some strengthening before attempting it. I have a bad ankle and need to modify some of the moves so I don't hurt it
Female, 28 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 9/3/12 = 164.9 (net: -50.3 lbs.)
Current mini-goal: 160 lbs.
Next mini-goal: 150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130 lbs.
"You don't have to change your life today. You only need to change your day today."