You don't need a car. Motoconchos and taxis will take you where you want to go -- even to the airport at El Portillo (usually costing RD$120 per person). Taxis and motoconchos are constantly traversing Las Terrenas, and all you have to do is hail one.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.