we just returned from a cruise that included a stop in Dubrovnik, to which we took the shuttle bus from the ship ($5 each way) and toured Dubrovnik on our own. In order to walk on the City Walls we needed to purchase a "Dubrovnik card" for 130 kuna. This card could be purchased at Rector's Palace or at the Official Tourist Information center outside the old city walls. Both places told us they would only accept kuna but that we would be able to pay with US dollars or Euros or credit card at the other place. The place to get tickets to walk the walls did not sell the Dubrovnik card (which included admission to eight sights, of which we went to three -- walking the walls (70 kuna), maritime museum (40 kuna) and the Rector's Palace (I do not know what the palace would cost on its own). The other sights whose admission was included in the Dubrovnik card were: Natural History museum, home of Marin Drzic, Art Gallery, Dulcic Masle Pulitika Gallery and Rupe Ethnographic museum. Suposedly the card includes free usage of public toilets. (The weekly Duvrovnik card costs 220 kuna and includes 20 public transit rides as well as the free entrance to those 8 cultural institutions. The art museum is outside the city walls and the maritime museum is entered from walking the walls.
We had to go to the ATM to get kuna to buy the card. Then we had to take the card to a place 50 meters from the entrance to the city walls to get it validated, and then get it validated again just outside the city walls and again inside the city walls. When we expressed surprise at these steps, we were told by one of the validators that this was a new idea from the mayor. We spent more than one hour going from place to place to try to buy the card as we received incorrect information about how and where to purchase it.
Some restaurants and stores accept dollars, euros and credit cards.