If you’re staying in the metro area you can hop on the Tren Urbano (Urban Train), combine it with the bus system in order to explore some of the metro area at a low cost. Tren Urban is used primarily by commuters. With some planning you can get around the capital city of San Juan. In order to experience Puerto Rico we highly recommend a car rental, you can make a bid for as low as $15 per day through Priceline and enjoy more freedom to explore the city and the rest of the island.
Please visit our sister site with more information on the Tren Urbano in San Juan Puerto Rico.