The Frommers website has this to say about some of Costa Rica's beaches:
With more than 1,200km (750 miles) of shoreline on its Pacific and Caribbean coasts, Costa Rica offers beachgoers an embarrassment of riches.
Santa Rosa National Park: If you really want to get away from it all, the beaches here in the northwest corner of Costa Rica are a good bet. You'll have to four-wheel-drive or hike 13km (8 miles) from the central ranger station to reach them. And once there, you'll find only the most basic of camping facilities: outhouse latrines and cold-water showers. But you'll probably have the place almost to yourself. In fact, the only time it gets crowded is in October, when thousands of olive ridley sea turtles nest in one of their yearly arribadas (arrivals).
Manuel Antonio: The first beach destination to become popular in Costa Rica, Manuel Antonio retains its charms despite burgeoning crowds and mushrooming hotels. The beaches inside the national park are idyllic, and the views from the hills approaching the park are enchanting. This is one of the few remaining habitats for the endangered squirrel monkey. Rooms with views tend to be a bit expensive, but many a satisfied guest will tell you they're worth it.
KAPOORS ON THE ROAD
We really didn’t want to travel overland too far from San José, and the hotels at Manuel Antonio were already full, but we found availability at a secluded resort not far from the entrance to the Manuel Antonio National Park. I really liked the looks of the buildings; Clandestino Resort has only twelve rooms, located in three thatched-roof bungalows arranged around an inviting pool.
We arrived just as the sun was setting and were surprised to see a photographer on the beach taking pictures of a bride and groom. When we pulled up to the reception area, we could see that all the guests were gathered in the open-air bar and restaurant. Our host apologized for the fact that she had not been able to contact us in advance about the wedding. She told us the bride’s family had invited us to join in for drinks and hors d’oeuvres.
We had a great time sitting in the shadows on comfortable loungers, watching all the young people dancing and celebrating together. We indulged ourselves with a few gins and tonic and snacks on all the munchies that were served to us by waiters circulating around the pool area. We were tired from the long drive and turned in early; surprised that the loud dance music didn’t seem to seep into our room to keep us awake.
We spent the next three days relaxing by the pool, taking long early-morning walks on the beach and napping during the heat of the day. We could have taken some excursions, but we were much more interested in staying put and enjoying the peace and quiet. Perhaps the indolence would have been too much for a longer stay, but for three full days, it was perfect.