When we were in St. Kitts we walked to town, very close to port, we enjoyed the "locals". We stopped at a beautiful church there and just walked the town. Our brother and sister in law took a snorkeling tour and said it was the best they ever did. I'm sure others on this board will have some options for you. Enjoy your cruise.
I'd suggest booking something in Antiqua, not really much by the port, you can go into town, but we noticed a lot of locals tend to want you to take thier cabs, etc...This time we booked a ship excursion and are doing the mini boats and hidden beaches, sounded like fun.
In St. Kitts, we went into town and did get a cab with some other cruisers and went to Turtle beach, quite a long ride, but very scenic. We had arranged for the driver to come back to pick us up at a certain time, and he was right on time. We all enjoyed it.
Marie, the snorkel tour was through Princess, in fact, they used the cruise personalizer and the tickets were in their room when they got on the ship. A great way to go instead of signing up on board. Hope I helped a little.