The Southern Caribbean cruise is one of my favorites. After 4 times I still never tire of it and when I can't decide which one to go on I always go back to this one. The ports change from time to time but always offer a great choice. While you are in San Juan enjoy the free buses that can take you all over with beautiful views of the Port and also go to the forts! The locals are always willing to share information and history if your interested.
Have a fantastic time!