Brimstone Hill is worth seeing - very historical, great views and well kept. The secret of enjoying St. Kitts is to get a tour that takes you away from Basseterre, which is not the most picturesque place.
We arranged a private tour - see our posting on the Port message board. The thing about Aruba is that it is much thinner on forts, museums and historical things, but it has this big Palm Beach tourist areas with malls, famous restaurants, hotels and a great beach with chairs to rent. Not much Caribbean flavor but convenient for all water sports and active fun.
There is a LOT of shopping in Curacao that you can walk too right off the ship. It is a safe and quick walk, and you can either take the pontoon bridge or the free ferry to the other side of the city. We only planned to enjoy the architecture and have lunch at Mondo Bizzaro, a cool local restaurant. Next time, we'll take the whole island tour.