My daughter (25 yo) and I (61 yo) were on the 2nd Southern Caribbean cruise last September and these were the Disney excursions we did:
Antigua- "Castaways Beach Escape" -This one took us to a beach called "Castaways." It was an okay excursion...included transportation, beach chairs, a tropical drink, and lunch (which was just ok). I think if you wanted an umbrella you had to pay $5. The beach was nice and the water was crystal clear. We probably wouldn't do this excursion again if we ever go back just because we did a beach excursion in Barbados too and that was much nicer than this one. The one thing I didn't like was getting to the beach they cramped like 10 of us in a very small van. But, if you like the beach, I would recommend this excursion.
One these cruises we went to St. Lucia instead of Martinique...and I really wished that they would have gone back to St. Lucia because that was our favorite island!
In Grenada, we didn't do a Disney excursion because my daughter wanted to go see St George's University Vet School...but we talked with some other people who did the "Best of Grenada" tour and they loved it! Grand Anse beach looked like it was very pretty when we rode by...Grenada is the spice island and they have a market there where they sell different spices.
Barbados: "Barbados Beach Break" -Another beach excursion and it was wonderful! The water was crystal clear and they provided beach chairs and an umbrella...no lunch or anything but they did give everyone a tropical drink when we came in. My daughter brought her snorkel with her and she was able to see different fish and even a huge sea star! There were guys there selling tickets to go snorkel with sea turtles...I think it was $15 but to be honest my daughter really wanted to do the Disney excursion so she was considering it. We watched them take them on the boat out and noticed that where they were going wasn't that far away and after talking with some other people I am glad that she didn't do it because the turtles weren't in their natural setting...the reason why they were there was because the boats were feeding them...but if you want to go swim with sea turtles and don't mind that sort of thing I would recommend it! But the beach was fantastic! If we ever go back we will definitely do this excursion again!
We didn't do a Disney excursion in St Kitts, because where you dock is downtown and you can walk around and shop. It seemed like every port we went into had a nice shopping area close by to walk around. We really enjoyed this cruise and hopefully you will too! Hope this gives you some ideas!