The town itself is small enough to cover entirely on foot — twenty minutes from one end to the other. For the beaches north of town, the choices are: rent a bike (good for the closer beaches like Resende and Tiririca, hard work for the more distant ones), hop on one of the regular minibuses that run along the BA-001, take a moto-taxi, or hire a car.
A useful local trick: the same minibuses that serve the beaches will drop you at any access path on the BA-001 if you ask. Flag one down at the small bus stand on the road north out of town. Fares are minimal.