On one of many trips to St. Thomas, we went over to St. Johns and rented a car to see the island. It is a very beautiful island and one of our more memorable experiences.
I'm not a fan of San Juan, but on one of our trips with this port, I took the rain forest tour. I enjoyed it, but be prepared for a 1 hour ride each way.
I have only been to St Martin a couple of times but it is one of my favorite stops. This past January when we were there, my daughter and I went kayaking and snorkeling.
I have not been to Samana yet.
Enjoy your cruise. We are headed out in another month for cruise #3 this year.