MICK - these two islands have some of the most beautiful beaches!.
You can do it so easily on your own.
St. Thomas - take the time and make the little extra effort to get over to St. John' Island - it is breathtaking, Trunk Bay Beach has been voted one of the most beautiful beaches by the Travel Channel - you have to take a ferry to get to St. John Island - here is a link full of great info: www.vinow.com
. Trunk Bay is a bit touristy for our tastes, all the cruise line excursions end up here, we really like Cinnamon, Hawksnest or Honeymoon.
On St. Thomas proper there are also some really nice beaches . Meagan's Bay is the most popular - again a bit of a tourist trap -imo - no snorkeling here if that is a consideration. Coki Beach is also very nice, as is Emerald.
Do the beach in the am, go back to the ship, drop off your beach stuff, change, grab a bite to eat and then head out to do some shopping- you can shop at either the pier or graba cab and go downtown. IF you have some time take the Paradise Point Tramway up the mountain for an amazing view of the harbor.
St. Maarten - Orient Beach is the most popular beach, lots of goings on, very pretty beach - one section on the far end is clothing optional its clearly marked. You can get there by cab very easily. Dawn Beach is another great beach, a bit more laid-back than Orient - can be a bit choppy at times. We did an excursion when we were in St. Maarten last time - the Golden Eagle Catamaran sail - you sail on this large boat up the coast - which is gorgeous, you sail over to this small island refuge Tintamar Island - here you can swim, snorkel ( they feed the fish) or just relax on the beach. Food and drink are provided. Its a fun trip, on the way back there is dancing, singing and champagne - toasting another successfull voyage- great time. St. Maarten is another great shopping port. - on the Dutch side - you walk down the pier, grab the water taxi into town, its really cheap like $5 for an all-day unlimited fare. Then just wander around town. We feel the shopping St. Maarten to be a bit more relaxed and laid-back than in St. Thomas.