Post back surgery, looking for new low-impact cardio

In July of 2011, I had an artificial disc installed at my L4-L5 disc level. In January of 2011, I had another spinal injury that has caused me a lot of pain and disability. I had finally gotten to a point where a light jog was possible before January. Now, that's not as easy to do because I have tingling and numbness in my foot after a lot of activity. Last summer and fall, I was going to do lap swimming for cardio and rehabilitation, along with pilates for strength building. They closed the pool last fall because of military budget cuts, so that's not an option for me anymore.
Does anyone have any good suggestions for LOW IMPACT cardio? Lately, every time I do a good, solid cardio workout, I pay for it with 2 days of leg and back pain/cramping. I need to up my cardio frequency, but I don't wan to risk further injury and pain.
I saw the "DDP Yoga" being reviewed ... basically, just yoga with constant muscle tension... but I'm not sure if that's any good or not.

Stationary bikes are also out because of the pressure on my SI joint/nerve which is where the new damage is affecting.
I do 15 minutes of Zumba twice a week, but more than that has caused a lot of leg pain/discomfort
Walking is okay, but I feel like I could be getting a lot more bang for my buck.

I refuse to let the injury or surgery be the reason I don't workout, but I would really like to find something that won't leave me stuck in bed with an ice pack for two days.

TL;DR:
I had a major back operation and subsequent injury that prevents me from doing high impact cardio.... they closed the pool, where I was doing cardio, and now I'm struggling to find a replacement workout to burn some calories!