A quick speedboat ride from the city center, Islas Del Rosario (Rosario Islands) is the a great day trip to the beach. The beaches are beautiful, with the calmest, clearest and most peaceful water. It feels like you're at a private beach paradise for the day because the whole resort was quiet and idyllic. We went twice during our week in Cartagena because we loved it so much. Reasonably priced for a full day out with lunch, beach, drinks and a massage (if you wanted one). I want to go back now.