Well, it depends on ages involved. For kids, Coral World at St. Thomas or the Butterfly farm at St. Martin. Either can be done on your own easily, and cheaper than thru
DCL. In addition, on your own, you avoid the crowds.
When you get off the boat, there is a tourist info area. At St. Thomas, they have a small magazine (free) with coupons for Coral World discounts. Take a taxi to Coral World. It is an interactive sort of aquarium. There are hiking trails, an underwater observation area, a "normal" sort of aquarium, and a "hands on" area where the kids can touch a nurse shark and other small creatures. Take towels from the ship as directly across the "street" (more of a parking lot area) is a nice beach. Then catch a cab back when you're ready. There's also a snack bar and gift shop. If you do it thru DCL, you will have plenty of time at Coral World, but no real beach time, and it will cost more.
Butterfly farm at St. Martin--same sort of thing. Get a cab from the tourist center near the dock. Since your timing will be different from the excursion group, you will find the place mostly deserted and have all the time you want to look around. On the way back, the cab will drop you either downtown for shopping (15 minute walk back to ship) or take you to the dock. The drivers do a tour speech along the way.