You can do dubrovnik on your own. the town is quite small and easy to do alone.
at istanbul there is a 'hop on hop off' bus at the port gates 20 euros to go around the whole town. can be windy near the water depending when you go so be prepared.
santorini is easy to do alone too, walk up to the town of fira or take the cable car or a donkey ride if you are brave enough.
at venice take the shuttle bus to piazzale roma where you can catch a vaporetto which will take you down to san marco. buy an all day ticket and you can get on and off anywhere you want to.
at naples you can take the train to herculaneum or pompeii, beware pickpockets here.
dont know kusadasi samos or zakinthos but hope the other info is of help
any other questions i would be pleased to help judith.
Cunard usually do provide free shuttle buses to city centres if said city centre is more than a mile or so.. In Naples you can do your own thing easily whether you want to go to Pompeii, Harculaneum, (by train), Capri or Sorrento by boat/catamaran or there are 3 routes that the open top city tour buses do from just behind the castle right next to the port. In Venice use the vaporetto (water bus ) to take you to St Marks Square then you will be well placed to walk everywhere on the main island of venice. The Vaporetto will also take you to Burano and Murano if you wish to see Venetian glass being made or buy lace items. There is too much to do in Venice in one day so do your homework and plan it all beforehand. Dubrovnik is a beautiful city just to stroll around, Zakinthos - go to a beach or stay onboard, Kusadasi -Ephesus is a must see place, Never been to Santorini or Samos but in Istanbul just follow the crowds to the bazaar. Hope this helps. You will love the ship.