the "must do" is a swim with the Sting Rays, most have a snorkel stop but you don't have to snorkel if you don't want to. To interact with the rays you stand on the sandbar and they will swim all around you, amazing creatures. Book independant google grand cayman cruise shore excursions to see what is available.
7 mile beach is very nice, there is a Turtle Farm that is interesting.
Another excellent option is the Cayman Island Jeep Safari tour you go all over the island, see the cliffs at Pedro's castle, sample rum cake at the rum cake factory, go to Rum Point and so much more. Its fun in a jeep and our guide was excellent
We did the Off Road Jeep Adventure and loved it. We explored the entire Island with a tour guide in the lead jeep. Each jeep had a two way radio so we could all hear him and could ask questions as we went. We went to a beautiful resort to use the beach and pool and get a cold drink and then went to "Hell" and ended with an hour or so on Seven Mile Beach where they served lunch. You can get this excursion with or without lunch. You are finished around noon and could return to the ship for lunch.
Whatever you decide to do, just have fun!
Carnival Fantasy (15 Member Family Cruise)