Half moon cay is not very large so I assume that is why there arent many excursions to choose from. However, it is BEAUTIFUL, and as it is a carnival and holland america owned island, they set up a wonderful feast (free) for you to pick on all day right on the island, with huge grills, all types of salads and goodies and beverages, with tropical music playing (CD). The beach is so clean and white..fine as sugar, and the water turquois blue. You are only there half the day, so all hubby and I did was enjoy the beach,have an awesome tropical meal courtesy of carnival, and took a nice stroll down the paths near the beach while enjoying the flora and fauna. I dont know if they do this on all the cruises, but for your sake, I hope so!
Ive been to grand turk, and for me, it was a huge disappointment. Not much there.