there isn't any excursion, you can single out. It depends on your group's interest. I would split them, and get the less active ones on a bus tour, which usually is about 4 hours, and let them afterwards walk around in Philipsburg for shopping. The active ones, I would put on the so called Rhino tour, which are small inflatable boats for two each. They go on a guided boat tour in the lagoon in their self driven boat (approx. $100 per boat). Or put them on a glass bottom boat tour with the Sea Explorer in Grand Case, or do a snorkel tour with them.
If all these options are too expensive, bring them to Orient Beach, or Pinel Island ($5 for the ferry), and let them enjoy the beach. More on www.st-maarten-info.com under 'Activities'.
Enjoy your trip!