Ahoy, Lisa!
I can't blame you for wanting to do some serious shopping in St. Thomas, I bet it will be fantastic! Since you are looking to power shop, you might want to skip a Port Adventure that day. When you do an organized Port Adventure through Disney Cruise Line, you are at the mercy of the transportation drivers.
If you would still like to try to do some sightseeing, I would suggest talking to one of the local taxi drivers. We have had very good luck by asking the drivers where a good beach is and having him take us there. At one port, the driver even returned a few hours later to take us back, and he was on time! It was wonderful. This allowed us to go to a beach on our own time, and then return to the port area to do some shopping and get some Caribbean "Punch" to drink.
If you decide to try a Port Adventure, I have a friend who just returned from a 7-Night Eastern Caribbean Cruise and he said that the
Magens Bay Beach Break was awesome! It is now on my list for my next Eastern Caribbean Cruise!
I hope this helps with your cruise planning. Please visit the Disney Parks Moms Panel with more questions as your voyage gets closer.
Happy Cruising, Tracy V.