St. Kitts has never been one of our favourite islands but I would recommend a trip to Brimstone Fort. Very historically interesting and if you can handle all the steps to the top, the views are breathtaking.
Splendor of the Seas 1997
Regal Empress 1998
Sovereign of the Seas 2003
Princess Sun 2006
Princess Sea 2007
I absolutely love St. Kitts -- discovered it in January of this year and have been there twice since -- and going back the end of October. Check out the Scenic Train... it's the old narrow-guage sugar cane train that now transports tourists in comfort of a tour around the island. If you can do a combination rail & sail, that's even better -- you see all of the island from two different view points.
If you just want a day at the beach, go down the penninsula - Cockleshell Beach is well known
If you just want to shop, the port area has some very nice shops & restaurants right at the ship.
Be advised the Cockleshell beach is at the very southern tip of the island, so you are in for a long and expensive cab ride. On the plus side you do pass through some really neat scenery where you can see both the Atlantic and Caribbean at the same time.
All I ask is a tall ship and a star to steer her by; John Masefield
Carnival: Glory 2004, Destiny 2008, Splendor 2009, Freedom 2011, Valor 2012, Dream 2013
Celebrity: Summit 2011
Princess: Ruby 2010, 2014, Caribbean 2013, Coral 2014, Regal 2014
Star Clippers: Royal Clipper 2015