St Maartin is an easy place to rent a car. It's fairly inexpensive, they drive on the right and the rental agencies are right on the pier. There are many great reataurants and a butterfly farm on the French side. Just try to avoid the area right downtown in Phillipsburg, and watch out for farm animals in the road. Think 'Sunday drive' not commute.
The rain forest in San Juan is supposed to be spectacular, but I haven't made it there yet. Seem ever time I get there we don't dock until late afternoon.