Hi ncara,
We just returned from a Med cruise on the Millenium and were very impressed by the ports you are interested in. Every one seemed to be greatly impressed by Dubrovnik. We had a shuttle that ran from the dock side to the Pile gate of the old town. The old town is a walled city with no motorized vehicles, and is a beautiful very well maintained city. The city wall is about a mile around and there are tours that provide a walk around the top of the wall. Many nice outdoor spots to get excellent drinks and food. A market place and many interesting shops and buildings. Prices are reasonable and local craft items are well made and in a variety of types. This is a real high light of the cruise.
In Santorini we took the tram up to the top of the cliff from the tender dock, and walked all through the village. Narrow streets with lots of steps and slopes. Many interesting shops and points of interest. We stopped for a cold beer and snacks at the top of the cliff with a view of the harbor and our ship anchored off shore. If you are so inclined you could walk up/or the 800 odd steps to the top, or take a mule ride up the same steps. Some very beautiful sights. We also tendered in to Mykonis, and spent the day walking around the harbor, and enjoying the sights.
Peter