The Rosario Islands or Islas del Rosario (in Spanish) are just 90 minutes travel by boat from Cartagena. The islands are a great place to get away from the bustle of that beautiful colonial city. The few beaches are lovely, and the numerous islands quite mesmerising. This video is of a full day boat tour setting off from Cartagena, where the video begins. It gives a good feel for what the Islas del Rosario are like. The final shots of the video are at Playa Blanca, on the mainland (I believe).
While this video shows the possibility of a day tour to the Rosario Islands, I'd suggest you spend a couple of nights there. See the section of beaches in Colombia for more advice.
If you have a Rosario Island for sale or own real estate on the islands, by any chance, please email me - I'd like to buy it! It's that beautiful.
Rosario Islands: Colombia's Rosario islands, or Islas del Rosario, have excellent beaches.