Grand Turk is a very clean, very flat island. There were some posts about the dune buggy rentals here, but don't remember whether it was through Carnival or Costa. Anyway, they said they had fun, but there wasn't much to see. However, I remember a post about the Segways that they said it was a ball. Grand Turk got pretty wiped out by a hurricane not too long ago. There's a nice beach right where you dock and there's a shopping area there, too. They have a downtown area that isn't too far and I think, if I'm not mistaken, that's where the Segway tour went.
We did a snorkel trip through Costa that was great. We stopped in 2 areas, one for coral and one for sea life. The only problem was they crowded too many on board for the trip, but the water was fantastic. Crystal clear with lots of beautiful coral and colorful fish. There was a 'cuda that hung around, but didn't seem to mind us and didn't seem to be hungry
I hope you enjoy Grand Turk, I sure did.