There's a train on St. Kitts that was used in the sugar trade that now takes tourists around.
As suggested Nevis is a great little island, they have a Heritage Trail that takes you to many of the notable locations, the island is very small so you can drive around it in no time. Lot of abandoned ruins from the sugar trade that you can just stop at and visit on your own. Golden Rock Inn is a nice stop for lunch or tea, not unusual to see monkeys roaming around the grounds. The ruins of the Hamilton Plantation are there, Alexander Hamilton was born on Nevis; ruins of the first church to allow integrated services is another interesting spot, Qually (sp) Beach is a good place to spend the afternoon. Weekends you can try the goat waters, just about every place will have it. Killer Bees at Sunset Bar, are much stronger than you might think so beware. There's a natural hot springs you can walk to from the main town. Great views from the top of the mountain, and not too bad of a hike. It's a nice quite island that doesn't get the crowds from the cruise ships, very friendly people. To give you an idea, we stopped there while sailing the Leewards and rented a car for the day, the fellow we rented from (Teach) wasn't going to be around in the evening when we would be heading back to the yacht so he told us to just park it on the road near the ferry dock and leave the keys in the ignition. They also have a very active sport fishing fleet there and hold a billfish tournament each year.