I don't have CP, but many of my students do have it, or they have similar issues. It depends on how involved you are. For some of my students, stretching is all they can do, and that with assistance. For others, swimming is something they really enjoy and get a lot of benefit from. I know one adult in California who participated in the Paralympics Swimming. She apparently sets records in her sport. I don't know as much about her swimming as I do about her assistive technology, since that is what we've communicated about.
Weight lifting (low weight, high reps) has been another thing that many of my male students enjoyed, although some of my female students have as well.
My gym has something called an Upper Body Bike that you can move with your arms, its a stationary bike. I used it for quite a while when I first started exercising seriously because I have nerve damage to my ankle... that lets out most forms of cardio exercise that I can think of: walking, standing aerobics, bike, treadmill, elipitical, stariclimbers, etc... I swim a lot for my cardio and do weight lifting.
I do ride a stationary bike, but that is for physical therapy as opposed to exercise.
What do you have access to, and how involved are you?
(I'm not sure of a better way to ask THAT question!)