Get lost in the Old Walled City
Obviously, the top thing to do is literally walk around and get lost in the streets of the old walled city. Don't leave until you have wandered every nook and corner.
Take a Game of Thrones Tour
The island of Lokrum holds the (replica) Iron Throne but there are tons of tours you can join in the old city.
An absolutely gorgeous island only 600 meters from Dubrovnik. I spent the whole day here but you can easily make it a half day trip. Read more on tips for Lokrum here.
Spend the day at Banje Beach
The beach has tons of things to do and a great restaurant onsite.
Walk the city walls
Obviously one of the top things to do in Dubrovnik is walk the city walls. There is no shortage of absolutely stunning views.
Get out of the walled city for dinner
Dubrovnik is pretty touristy, which means lots of restaurants that are overpriced and mediocre. Luckily, I made you a handy list so you can eat at the best places instead of the worse. LINK
Take the cable car to the top of Mount Srdj
Plan to take the cable car to the top around sunset for a truly magical evening.