Although your sons are non-swimmers, do they enjoy the water? Megan's Bay in St. Thomas is a wonderful place for non-swimmers. I know that some consider it touristy, yet we always have a nice time there, as does our son (also a non-swimmer). Alternatively, you could take a side trip to St. John, which is absolutely beautiful.
San Juan has wonderful forts which we found fun to explore. Are you doing a pre-cruise or post-cruise stay there, or is it a port of call?
In Aruba, now that the Natural Bridge is gone, I'm not sure what to recommend. In Barbados, we took a tour of a rum factory, which my husband liked, followed by a couple of hours on the beach. I wouldn't call this a "must see," though. My favorite memory of Barbados, though, is talking with the people who live there. They were very friendly and a pleasure to talk with.