We sailed the Eastern last year and then DH and I went back to the Vis for our 20th Ann for a week.
As far as excursions - you can have a good time and save a good bit if you do it yourself.
St. M. Look at all that is offered and if something sounds good you can get a taxi to "it". Most people prefer going to the French side.
In St.T I highly suggest going to St. John on your own.
Plan to get off the ship ASAP. Grab a taxi to Red Hook, Ferry to Cruz Bay, Taxi to Trunk Bay. There is a lot to see in Cruz Bay. We ate at a great Mexican place for lunch. My sister, her DH and two DDs did this so I know you can. In fact when DH and I did it while staying in St. T the ferry was full of families off a Carnival ship. You just need to allow 2.5 hours to get back to play it safe from when the ship sails (usually a 4:30 all-aboard so 2:00 in a Taxi at Trunk Bay.) If you get back faster you can shop at Heavensight.
Trunk Bay was one of the most amazing places I've seen. We liked several spots in the British VIs but you will not have time to do that.