Cartegena is one of the most interesting and intiguing cities in the world. The mix goes from 1500's forts, churches and city to 2002 high priced condos and hotels. I' m not sure that this is a city to get adveturous in. You have only a few hours and if you have not been there before you will want to see the fort, old city , Stjohn's church and LaPopa Monestary. In my humble opinion, this is the one place to go with the ship's tour. It will only be about $30.00 but will take you to see these few places and some brief shopping as well. Cartegena is as stated a beautiful place. It is also surrounded by abject poverty, a 30% unemployment rate, Aggressive street vendors and street beggars who are also quite aggresive. You will see armed guards at every stop (military in some places). Please don't let what I've said scare you, I am only suggesting that compared to many other cruise stops, this one is not as tourist friendly. It is a "must see" however.
Whatever you choose, I know it will facinate you.