We will be going to both islands in a couple weeks. We like the railway excursion in St. Kitts. We have done it three times! Last year we decided to go to the fort and enjoyed it very much, too. There are some steep steps to climb, but we survived!
We are going to the forts in San Juan. We have seen them before; but, like Peter said, they are very interesting and historic. Plus, you can see them from the ship, which kind of "draws" you to them!
All the forts are neat and you can't go wrong either way.