we don't usuallly do an excursion in St. Thomas. St. Thomas is very easy to do on your own. Beach and shopping are the most popular things to do.
It depends upon what you are looking to do.
Suggestions: IF you want a really nice beach with decent snorkeling then just grab a cab to Sapphire Beach. If you want a gorgeous beach with amazing snorkeling then you would need to get over to St.John Island by ferry, or book the Champagne Catamaran Sailaway to St. John Island - great excursion.. Another fun side trip is the Paradise Point Tramway - it is a short walk from Havensight Pier, you take a tram up the mountain, the view of the harbor is beautiful, alot of the island weddings take place here. There are small shops, restaurant, gorgeous flower gardens, birds.
If you dock at Havensight the mall is right there or you could grab a cab and head into downtown Charlotte-Amalie.