Have been to both islands....
St Maarten.... if you feel adventurous, one of the best tours is the America's Cup Race Challenge, which you actually will race another team in an official America's Cup yacht. It is alot of fun...and you will actually work the sails, etc. Sign up early, as it is a popular tour. It is also a shorter tour... lasting only about 3 hours so you will have plenty of time to do some shopping or go to a beach. The America's Cup Challenge is done right in the harbor, near the cruise ships, and shopping and some great beaches are right nearby if you don't want to take a cab.
St Thomas... I went scuba diving there.... but know people who have taken the ferry to St John's..... and loved that tour. Another quick (and cheap) tour is the tram ride up to the highest point in the harbor area... it takes you right up the mountain and will give you a great view of Charlotte Amalie and the harbor! It is within walking distance of the ship and near lots of shopping too.
We were in St. Thomas and St. Maarten a few weeks ago on the Radiance. We enjoyed our tour of St. Thomas with Sunny Liston (not a cruiseline tour,) and you can check this board or the internet for reviews and itinerary. The trip to Mountain Top, and the view, were awesome, but skip the Banana Daqueri's - they were overpriced and the bartenders were grouchy. We opted to go back to the ship instead of staying for the 2 hour beach stop at a very crowded Sapphire Beach, and enjoyed a quiet few hours poolside on the ship, with time to get ready for dinner. When we go back to St. Thomas, we plan to spend the day at Magen's Bay (a very pristine, undeveloped beach accessible by cab, small admission fee.) We were there a few years ago and this is truly what the Caribbean is. Sunny will take people to Magen's instead of Sapphire if everyone on the tour agrees. I also preferred shopping near the pier rather than downtown.
Your ship may stop at St. Croix after they leave St. Thomas for refueling. We enjoyed getting off the ship (it was after dark, so don't go alone) and walking to the little town. It was a beautiful, warm evening and there were street performers and vendors, who came out just for us. Several of the shops were also open.
We took a bus tour (cruiseline excursion) around St. Maarten, since it was our first visit to the island. We stopped at Marigot (French side) for about a half hour, then on to Phillipsburg (Dutch side.) Unless you are dying of curiousity to see the French side, I'd recommend using the reasonably priced water taxi from the pier to Phillipsburg, where you will find jewelry, linens and souvenirs all considerably less expensive than St. Thomas. It was a much more pleasant shopping experience, and there is a beautful beach right nearby. The water taxi is so efficient that you could easily shop, take your stuff back to the ship, then go back to the beach for several hours of r and r. Or, if you are a family, let the shoppers shop and the beachers beach it! They have chair and umbrella rentals everywhere. Phillipsburg is a little jewel.
Whatever you do, you can't go wrong. These are two great ports.