Uvita, Costa Rica, is a magical place with a huge beach perfect for a long stroll. The famous "whale tail" is a natural formation where the sand (and rock) sprawl into the ocean in the shape of a whale's tail (as seen from above).
You'll share the beach with others, as this is part of a national park, however, the sheer size of the beach will leave you feeling the solitude.
The best beach in Costa Rica, en mi opina. Go to Manuel Antonio for the water. Go to Marino Ballena for the beach, the mountain views and the long stroll.
Stay in a nearby hostel to enhance the fun.