I'm sailing on Liberty of the Seas in two weeks hitting all the same spots (3/1/08). In Labadee, the zipline is a fun as well as just sitting around the beach on a floating matt. In St. Marteen; we're doing the snorkle via caterman; and in Puerto Rico, we'll head over to Bacardi for the tour. I'm also 29 and my buddies and I have done these islands before and found the tours to be a good time.
In St. Maarten, you may want to consider doing the America's Cup Regatta, a once in a lifetime expierence, you'll love it if you want to try sailing in a race. Your excursion is right there by the cruiseships. One time we also did some Rhino boats that was great fun as well.
If you want to take some time to travel in St. Thomas, take the ferry over to St. JOhns for some absolutely beautiful beaches, great snorkeling, etc. It does take time, but well worth the trip.
In San Juan I would grab a cab and go up to El Morro and then wander your way back down through Old San Juan. The fort is at the top of the hill, and it is easy to walk back DOWN to the pier area. Frommer's online has a great walking tour if you want to check that out.
I agree about the America's Cup Regatta in St. Maarten. It is fabulous. You would have time afterwards to hit a close beach or go shopping, which is great in St. M.
St. Thomas - such a beautiful island, and St. John is gorgeous! It is easy to do St. John on your own, but it does take time to get there and back. On St. Thomas, Sapphire is supposed to be beautiful and a great place to snorkel. There are many beaches there, and the taxis by the pier are plentiful. There is also the tram that can take you up for a great view of the island. It is really close to the pier, and you can do it on your own.
Have a great cruise with your mother. Oh, the fun it is to make memories!
Mariner of the Seas 3/30/08
"Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover."