On St. Maarten, my mom and I did the French Riviera Beach Rendezvous (Orient Bay, French side). They bus you over to the French side of the island, to Orient Bay (which, unfortunately, people are allowed to walk around naked). The beach is beautiful however. You're provided with a complimentary drink and lunch and your own beach chair (and umbrella if you wish). This is more of a laid-back excursion...but if you wish for relaxation, this one's great!
On St. Thomas, we did the Water Island Mountain Bike Adventure, which I LOVED! I'm more into the active excursions...and on this one, I felt like I was on my own adventure to explore the island! They take you over on a water ferry to Water Island, giving a brief history lesson on the way over. Once you get there, they bus you to the top of a mountain, where they then fit you for your helmet and mountain bike and you're off! MOST of it it down-hill, but if you like a bit of a challenge, there are a few up-hills. After the bike ride (about an hour or so), you can choose to relax at Honeymoon Beach (or continue on for lots more up-hill stuff). We chose the very secluded beach which was beautiful.
At
Castaway Cay, we chose not to do any excursions. There are plenty of water toys to rent (tubes, mats, paddle boats, etc). Plus you can rent bikes, you can parasail, or you can do the banana boat ride (which I've heard is great fun!). Whatever you choose to do, relaxing is the key at Castaway Cay since it will be your last day.
Hope this at least gives you an idea of what we did..and helps you choose your excursions!=)
Mickybean0816