We just love St. Maarten. If you are not looking for anything too exotic, I would suggest taking the water taxi across to Great Bay ($5.00 per person for all day, as many trips as you want) and you can browse the shops and sightsee a bit and them relax on the beach. The locals are very friendly and duty free prices are very low, possibly the lowest in the Caribbean along with St Thomas. You can rent to nice beach chairs and and umbrella for $5.00 and the waiters at the nearby bars will serve you right in your beach chair, and cheap too.....usually the beer of the day is $1.00.
If you want to go further afield, then Orient Beach is beautiful (if you don't mind nude bathers) and lots of great shopping and restaurants nearby as well.
Friends of ours took a taxi to the beach right near the airport, where the planes fly right over your head, but this would not be for the faint of heart, although they loved it.
Another suggestion I read on here was to hire a cab/do a tour of the island. I dont think its that big. You can visit the butterfly farm, etc. Check out what your cruise has to offer as well. We had planned to go to Maho Beach to watch the planes but our son came down with a fever and ear infection so we missed that. We are planning a trip to St Maarten on our own now.
they are doing Major road re-construction in St. Maarten - i would not want to rent a vehicle and go it alone- hire a cab and explore. We chose a great excursion - The Golden Eagle Catamaran - you sail up the coast (beautiful scenery) over to a small island preserve where you can splash around in the water, stroll the beach, snorkel, get a mud bath and just have fun. Do the morning excursion - go back to the ship, grab a bite to eat, change clothes grab the water taxi into downtown and shop, shop to you drop.