St Kitts is ok. Not much there other than shopping. BUT I HIGHLY recommend taking a taxi to go Brimstone Hill to see the fortress. It was the most extensive British fortress I have every seen. Very well preserved and very interesting. If you get off the ship and walk into the market area. Go right once you are off the pier area and you will see a taxi depot. There are signs listing prices to go to various spots. We negotiated a bit with our taxi guy and he gave us a mini tour and took us to the Fort and waited there and brought us back for a very reasonable price. You could also take a ferry to Nevis depending on your time in port.
St Lucia- Some awesome beaches there. There is a Sandals resort but I don't know if they have day passes. We went on a pirate ship ride/snorkel.
It was an all day tour so I don't have much more to say about the island.
Everyone was friendly on both islands so you shouldn't have an issue there. If you don't want to book a ships excursion, we had no problems finding a "private" taxi to taxi us around for the day with a bit of haggling on price.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007