I went to the Rosario Islands and Playa Blanca yesterday. I booked a day trip and since the beginning of the day, when I took the boat with the rest of the people, I felt to be treated like caddle. The islands are 27 but 24 of them are private island, then the boat stops in one of them to visit the aquarium, which is not worth because there are about 10 diferent kinds of fishes that you can see in other parts of the world.
Playa Blanca is one beach of white sand and blue sea like other beaches in the Caribean Sea, and it has all the ingredients like in the most touristic places of this area. Lots of people try to sell to you everything and if you want to have a rest you won´t be able of it, they want you to give them a few pesos colombianos! My advice is that if you are coming to Cartagena, you can save the money, about 20 euros, and avoid this day trip. Go to other places of this nice country that it is worth! Above all because of his people!