I have been living in St. Martin for six years, I always tell my friends to stay at least one whole week here for vacation. French side is not that crowded as Dutch side as lots of the tourists come from cruise ship that only allows them to say just one day.
13F is right, drive by yourself is very easy, the island is not big and you won't get lost, just one main road and see around on a day that is also a pleasure trip and then make stop to Orient Baie (there are a few nice restauant in front of the nice beach, all are good food and good service. Or anywhere you want to try the food. People here are very nice to tell you the road and give you direction. Remember: Drive slowly, you will come across cows or goats on the road, even people make stop on the road and just say hello to the cars. This is an island - not a city life. But overall. But if your time is tight, hire a taxi and get info beforehand, know where you want to go and don't let them to bring you what they want, and choose your restaurant. Oh, if you can stay overnight, go dinner at Grande Case, they have very very good restaurants in the main street, walk along there, + Lolo (BBQ hawkers center, quite interesting). Enjoy.