As for San Juan, PR
, I went on the "Walking Tour of Old San Juan". I really enjoyed it. However, this is a shore excursion for physically fit people. If I knew how much walking I was in-store for, I would have practiced for weeks on a treadmill at my gym.
This is also a shore excursion for people who are fascinated with old and historical things (as I am). You get to see parts of San Juan that are up to 500 years old.
Another nice shore excursion to do in San Juan is the tour of the Bacardi Rum Factory; or so I heard.