Lots to choose from - lots of opinions - for starters, on Thomas, Coki Beach next to Coral World makes a nice combination of an afternoon, and Secret Harbor.

On St. John you have the Reef Bay Trail on Trunk Bay, and Cinnamon Bay are both great options.

St. Croix - I leave that up to the locals over there............

We loved Maho Bay on St John, and Waterlemon Cay on St John. Really you cannot go wrong on any of the beaches of St. John. Some are a little more crowded then others (Trunk bay gets the cruise ship people).