We've done Costa, NCL (Seaward, Norway), and
DCL's Magic-3 times(Eastern -1, Western-2)! Each of our Magic cruises has been different. We've tried new things each time, and as our DD (now 10) gets older, the experiences have been different as well.
When DD was 5 & 6 we enjoyed visiting Key West. We went to the Butterfly Conservatory, Aquarium, Hemingway House, and the stops along the way via the Conch Train/Trolley. Who knows what we'll do this time!
Grand Cayman we've gone to a beach, did a tour of the island, saw the old Turtle Farm, and did some shopping. A lifetime ago, DW & I snorkled. This time we're thinking about the Pirate ship that sails in the harbor.
Cozumel...shopping. DW enjoys shopping in Cozumel. Best of all she buy real vanilla for baking. She bakes - I eat. We're happy!

We're thinking of maybe doing the Royal Dolphin swim next time, but we're still not sure. Only 11 months to make up our minds!
Castaway Cay is wonderful! We've enjoyed the beach, paddle boats, kite flying, Cookies Barbeque, I went parasailing, bannana boat rides, ...and of course the drink of the day...the Konk Cooler. I really don't drink except for the occasional Christmas party, or the obiligatory Konk Cooler on Castaway Cay. Next time DD & I are going parasailing-tandem! Can't wait!
It's hard to believe that I started this thread when we had 21 months to go! Now it's down to 11.