Located on the very south of Dalmatia, enclosed in it's nearly two kilometer long walls, Dubrovnik is considered to be one of the most beautiful towns of Mediterranean.
Whatever you decide to see there, you will not stop admiring the beauty of the place. Dubrovnik is not a big town, so 3 nights there will give you full experience. Get lost in a maze of little streets, have a capuccino on one of many squares inside the old town, visit the farmers market and just feel the smells of this amazing town full of history.
There are many things you shouldn't miss while in Dubrovnik, tiny island of Lokrum, located just across the city walls, Stradun street inside the old town, cable car to Srđ hill and fascinating city walls (because of the heat and crowd, be there there early morning or late afternoon)
Marina in Dubrovnik is located in Komolac, a bit outside of town.
But probably that is better, to avoid all the crowds.