We have not been to Dubrovnik yet but from my research you can do this port on your own.
Rhodes you can do the Old town on your own ...take the ships tour to Lindos or rent a car & do it on your own.
Santorini you can also do on your own
lost of people on Oceania book private tours in Istanbul & Kusadasi so maybe you can find people to share their tour with you
Enjoy the cruise
Alexandr Pushkin, Carnival-Holiday, HAL-(old)Westerdam, Orient lines-Marco Polo, Royal Olympic-Olympic, NCL-Dream, Dawn-Princess, ACCL-Grande Caribe, Oceania-Insignia (3), Regatta(4) Marina (1) St Lawrence Cruise Lines-Canadian Empress
Scenic River cruise June 11,2012