Playa Blanca is a nice, chilled-out little beach about an hour's boat ride from Cartagena. You can visit it with a day tour, but if you have time, spend a night out at the beach. It is rather rustic, and the accommodations are quite primitive... but it is also beautiful and quiet. We stayed at the Campamento Wittenberg, a friendly place with a little restaurant that put on a nice, extremely affordable dinner.