We dived around St. Kitts from the liveaboard Caribbean Explorer and thought that the diving was very good, although we liked the sea mounts around Saba better - but we had better weather at Saba so that was probably a big factor.
We spent a little time on land touring around St. Kitts and it was a beautiful place. I especially remember visiting some gardens where batik fabrics were being crafted while you watched. It was interesting to see the artistry used to produce the beautiful designs and fabrics and clothing articles were available for sale - expensive but unique and created by hand. It was hot in the gardens and workshop so you might want to go there early in the day, if you have the chance.
During that trip we cruised very near to the neighboring island of Nevis, but we didn't stop. If you have the opportunity to visit Nevis I understand that there is a small museum and other attractions for Alexander Hamilton (the guy on the US $10 bill that died in a duel with the US Vice President!) see the link below. I have become interested in Hamilton ever since seeing an excellent production of "Hamilton: the Musical" and would like to visit his birthplace and home if I ever have the opportunity to do so.
We were there during Halloween and visited St. Martin, St. Kitts, and Saba and I don't remember seeing plans for any Halloween activities, events, or parties on any of those islands. I don't think that it is a big deal in that part of the world. In fact, I remember remarking to my fellow divers on Halloween night "This is the worst costume party that I have ever attended!" (No one was in costume!) and they all looked at me in surprise, they had no idea what I was talking about.
There is a medical school on the island, so if you can find a bar that is popular with the students, you might get some Halloween action there - just an idea.
Have a great trip!
Walk in the Footsteps of Alexander Hamilton on This Tiny Caribbean Island | Travel | Smithsonian