The best way to get around St. Martin and St. Maarten is in a rental car. But really, outside of walking, just about any means of navigating between the French and Dutch sides of the island is manageable. Having a car will allow you more mobility and you'll find reasonable rental rates. Agencies are clustered on the outskirts of Princess Juliana International Airport (SXM), located in St. Maarten. Unfortunately, traffic can be pretty heinous on both sides. The island buses also cater to tourists and are fairly cheap to ride. If you don't like the idea of waiting on a bus, consider splurging on a taxi. And if you're feeling adventurous you might want to use a scooter; there's nothing like feeling the sea breeze in your hair.
Renting a car is a cheap and easy way to get around either St. Martin or St. Maarten. Some travel sites report rates starting as low as $35 a day. Rental kiosks are located just outside the airport. Don't worry about motoring between the two sides – chances are you won't realize you've crossed the French-Dutch border – but do watch out for traffic jams around tourist haunts like Marigot, Philipsburg and Grand Case.
Similar to the United States, you'll drive on the right side of the road when touring both sides of the island. Honking is considered a friendly way to say hello; don't be alarmed if you get honked at frequently. If you're driving on the French side, you'll need an International Driving Permit. Past visitors recommend you forgo the GPS rental and instead purchase a quality map from one of the shops on the island (according to some, the maps provided by car rental shops and hotels aren't extensive enough for navigating the island's roadways).