If you're looking for a quick day trip from Dubrovnik, I would highly recommend visiting Lokrum Island. It's a quick 15-20 min boat ride from Dubrovnik and costs 120 Kunas, which includes a round trip and entrance into the nature reserve (i.e. the entire island!) Lots of unexpected sites and hidden treasures.
Feel free to explore the inside of Fort Royal, entrance is free.
The peacocks love the attention and will put on a show for you, just wait and see
The Iron Throne from Game of Thrones! No lines and no fees to take a picture, what more could you ask for?