There's really no reason to take an excursion in any of these ports, unless it's something you really want to do.
For example, in St. Marten when you get off the pier, turn to the right and there are several kiosks where you can rent a car. It's a great and easy way to get around the island and the cost is inexpensive (from $35 - $75 depending on the time of year and type of car). We once rented a jeep for $75 and drove over the mountains on the dirt roads and it was awesome! But you can drive around the whole island (easy to get around) and visit several beautiful beaches.
On other islands, it's just a easy to get off the ship, talk to a few cab drivers to get some prices, and let them take you around the island. St. Lucia is beautiful and is definitely one island to explore.
But all the islands have some awesome beaches.
St. Kitts would be our least favorite as there's not much to do, but you can get around the island using a taxi for not alot of money (if I remember right, it was $25 each for a 3 hour drive).