Just returned last Sunday from these two ports plus Key West.
At Grand Cayman we did the Stingray City tour and really enjoyed it. At Cozumel we did the Mayan Ruins tour and found it very interesting. At the ruins if you can get Armando Chan as your tour guide you will find he is very well informed bieng part Mayan himself. Key West was chaos when we took the trolly tour of the city. Too many tourist and different tour groups got mixed together. Also got rained on and they would not let you open your umbrellas on the trolly. Go figure!
There is a tropical storm in the same area we were in last week. Hope you can avoid a storm as we were lucky enough to do.
I did the ruins tour in Cozumel as well. Booked it through the ship. If you're into history you'll enjoy it. The tour lasts several hours, though, and most of your time is spent getting to and from the ruins via ferryboat and bus.
Now posting as MichelleP.
In Cozumel we did a ships excursions (only way to go, much easier then doing it on your own) to XCaret on the main land (play del carmen). Its an awesome waterpark with snorkeling in an underground river, beautiful lagoons and beach and so much more.
In Grand Cayman we did the Stingray Sandbar...awesome..awesome..awesome.. Im actually going again in December and that will be on my list to do's...