Unless you are planning to hop directly onto a ferry to Vieques or Culebra, you should consider renting a car in eastern Puerto Rico. Even the destination resorts are fairly isolated, and you may appreciate the mobility if you want to get out and have a meal away from the resort or explore El Yunque or some of the great beaches. Rates start at about $40 a day, but it may be possible to rent directly from your lodging, so ask about packages.
From San Juan the east coast is accessible via Route 3, or Route 187 if you want to visit Piñones and Loíza. At Fajardo the road intersects with Route 53, which continues down the coast. Route 3 also continues along the coast, providing a more scenic, if slower, trip.