We did the same exact cruise for our 10th Honeymoon...er Anniversary (we call them honeymoons still since we feel like we are still on ours!). It's a great way to celebrate.
I used to live in Miami, and love Key West! It's an easy port to visit without doing a cruise sponsored excursion.
Our first cruise we took that called on Key West, we decided to just walk to a few museums - Mel Fisher's Maritime Museum, Truman's Little White House (our favorite of the tour), and Hemingway's House (mainly so I could see the cats!). We stopped at Margaritaville and Sloppy Joe's on the way back.
Our second cruise which called on Key West, we decided to rent a convertible and drive over the Seven Mile Bridge to Marathon, where we had lunch at Sunset Grille and Bar (which is right at the edge of the bridge). This was a bit more tricky since the only places to rent a convertible are either Tropical Rentals or at the airport. Tropical Rentals is obviously easier since they are right in town and will pick you up from the pier and drive you back...for free. The drawback is they close at 5:30 (and I believe the last shuttle back to the pier was at 5:00...although not as big an issue since they are in town and you can walk). However, the airport rentals require a taxi to and from the airport, which can double the cost of the rental easily and also eat up precious time. We went the route of Tropical Rentals and were able to drive up to Marathon, eat a late lunch, and drive back within three of four hours.
As for Grand Cayman and Cozumel, what are your interests? Beaches, tours, snorkeling? I find that Cozumel has great snorkeling...especially if you can find a tour that will take you out to Palancar...one of the best reefs!
Castaway Cay is great for just relaxing on the beach...especially Serenity Bay. We also love riding bikes on CC...they will let you rent your bikes at the family beach location and then return them to the Serenity Bay location. We loved doing this since it lets you get a nice island bike ride in, and then some beach relaxation without having to trudge back to the family beach just to return the bike. One thing I also love doing at CC is Castaway Ray's Stingray Adventure. Very cool experience.
On the ship, have dinner at Palo one evening. The food is excellent, but even better is the wonderful service and relaxing atmosphere. We never miss dinner here.
Hope this helps!