We parents just walked around Old Town (it's a short, easy walk from the pier). The young men took the ship's excursion "Nocturnal Canopy Tour Zipline Adventure." The ad copy claims "bioluminescent bugs and plants are alight all around you" while zip lining through the jungle canopy at night. They had a good time, but they didn't see a thing that was "bioluminescent," and our son said the course was a little tame compared to the other canopy tours he's been on. I note this "coded" language to that effect in the shorex description of the daytime tour: "This tour has been designed as a family experience although even those who have done ziplines before will find it an amazing experience."
We decided to take the no-brainer "booze cruise" approach to doing St. Maarten and went out with the Golden Eagle catamaran (through Celebrity). It was just what we wanted! They get lots of positive feedback on the Ports of Call>Caribbean>St. Martin forum, for good reason. The logistics are a snap--you meet on the pier and then walk through the cruise terminal out to a dock where the cat's waiting. The roundtrip sail is refreshing--you may lounge on the tramp or sit in the cockpit--and the snorkel/beach break is fun. There's not much coral or fish life to see, but don't miss the crazy fish-feeding frenzy! The libations flow on the return, and the crew works hard to make your day both safe and enjoyable. (If you've gone out with Golden Eagle in the past, as we had, note that they're no longer allowed to go to Tintamarre, the deserted island with the mud baths--it's an environmental protection thing).
(Reflection docked at Crown Bay, which is west of the more familiar cruise terminal at Havensight) -- I had looked at the online port schedule and decided with six large ships in port, we didn't want to compete for beach space at Sapphire, Coki, Magens, etc. So it seemed like a day on St. John would be perfect (the island is largely a US national park, and consequently much more natural and laid back than commercial St. Thomas, plus the beaches are gorgeous and snorkeling is excellent). However, I wasn't keen on the hustle and logistical hoops involved, plus with four of us, the roundtrip taxis on both islands + ferry fares were starting to add up to a lot of money just for transportation. So I did some research, and the #1-rated St. Thomas tour on TripAdvisor is a daysail from St. Thomas to St. John with Morningstar Charters. When I cross-checked them through CC's St. Thomas/St. John forum archives, Morningstar got rave review from cruisers. Long story short, we booked (very easy via email and phone), and they provided the perfect day! A taxi picked us up at the port and took us to their marina at Point Pleasant. They sailed two boats carrying six guests each to beautiful Honeymoon Beach in Caneel Bay, where we snorkeled with sea turtles--OMG, that was a first!--and also saw rays and all kinds of fish. They fixed us a tasty lunch (including veggie options) and generously poured the infamous USVI "painkillers," beer and whatever else we wanted to drink. The sail back was so relaxing...and then, it was time to return to the ship (our "all aboard" time was 4:45 PM). We were really happy with our Morningstar daysail and recommend it without hesitation. See http://www.morningstarcharter.com/ (when they divvied everyone up, we actually went out on their second sailboat, the Winifred, see http://www.sailwinifred.com/).