Everyone recommends Dunn's River Fallls in Ocho Rios, and I'm sure it's great, but if you want to do something that isn't in the company of 2000 other people, take RCCL's river tubing safari -- that was great fun, and there were only 8 of us! Other than that, I agree that it's a great place to stay onboard the ship -- the local merchants were unpleasantly aggressive, and a lot of the kids (including my 2 teens) were approached with offers of marijuana.
I spent my birthday in Grand Cayman, and it was the best birthday of my entire life! In the morning, we took the glass-bottomed boat tour, which was really cool. After lunch, we did the Stingray City snorkeling thing. Everyone -- just once in their life -- should pet a stingray! If you've never snorkeled before (I hadn't...), deep water in the Caribbean is NOT the place to learn, though -- I did better with my old swim goggles and just holding my breath underwater!
Have a great trip!