St Thomas - The ferry to St. Johns, for a visit to Trunk Bay is a good recommendation... but does eat up a bit of time.
The taxis at the pier in St. Thomas are mostly "group" cabs (open back decks to carry passengers, with set fees to different locations).
So, if you want to go Megan's Bay, just tell them at the pier, and they'll show you which "truck" to jump into.
A beautiful (and less crowded option) on St Thomas is Sapphire Beach. There will be "group taxis" to take you there and back as well.
For St. Marteen, I really like Dawn Beach. Again, less crowded than Orient Beach. Rental cars are the way we normally go there, but over the years, as more cruise ships visit, the roads have become quite crowded. So, if you choose this option, leave yourself time to get back in bad traffic conditions.