Best Southern Caribbean Excursions

[Spencer71]

Trying to plan our excursions. We're on Norwegian Viva and sailing out of San Juan to Tortola, Antigua, Barbados, St Lucia, St Maarten, St Thomas. Our group is two couples, about 50 yrs old with varied interests. Some like snorkeling, history, some not so much. Not interested in NCL's excursions, but will book through Viator or even custom. Want to keep costs down though. While I can look up things to do at each stop (and I will), I'm interested in what are the absolute best 3-4 things to do from all of the stops. What should we absolutely not miss? What should we book in advance? I figure we'll book a few things at some ports but then nothing at the other ports.