What to see in Dubrovnik?
Of course, Old town with city walls and many amenities inside. There are many locations where Game of Thrones took shooting.
Go to Srđ hill with cable car and see Dubrovnik Riviera from bird's perspective. Take 3 islands cruise from Gruž harbour and visit Elaphite islands. Go to island Mljet and visit National Park,also visit island of Lokrum near Old Town. Take kayak tour. Rent speed boat or sailboat. Or jet ski.
Visit small town of Ston on Pelješac peninsula and enjoy fresh oysters and mussels, Ston is famous about them and Ston has 2nd longest stone wall in the world, only Great Wall of China is longer. There is also many other things to do around.