A friend of mine (Steve) uses a personal trainer and he told me that one particular cardio workout he has been doing is a combination of squats and pushups. I think he said he is now doing 30 squats followed by 20 pushups, then he rests 2 minutes, then does a series of 5 more - for a total of 6 sets of squats & pushups with 2 minutes rest in between.
Since I can't run because of the plantar fasciitis at the moment, I thought I would give this a try. I started out with 6 sets of 20 squats and 15 pushups, but couldn't do the pushups on the last set. I was also sore as heck in my legs and chest the next day. It was a much better workout than I thought at the time.
So I think I will drop it back to 6 sets of 20 squats and 10 pushups; or I might even start with just 5 sets, and do it every other day. I think I'm supposed to gradually increase the number of squats and pushups each week. It definitely gets the heart going. It should also be good for strengthening my legs, knees, and feet for when I start running again.
Here is a good youtube video of how to do a proper squat: http://www.youtube.com/watch?v=QKKZ9AGYTi4
Here is a good youtube video of how to do a proper pushup: http://www.youtube.com/watch?v=Q2Wi3NUhriY