Georgianne,
Aerobics classes are at 8 am or so and around 4:30 pm. They last about 45 minutes and are great; however, you need to register. I did so on the first day, just after lunch.
Yes we did go to Palo and truly enjoyed the experience. However, I would suggest that you do it the last nite because you won't be missing any of the restaurants then. We did it on the third night and missed AP. Personally, I love Parrot Cay and would be very disappointed not to go.