An easy day trip from Dubrovnik is to the island of Lokrum. Because of its close proximity (a 15 minute ride), it’s popular amongst the locals and tourists.
Getting to the island is super easy. Just go to the old port, Porporela, and you’ll find different companies that will take you. A ticket is 60 kunas round trip and the boats run from every half hour from 9 a.m to 7 p.m. The island is tiiiiiny, less than one mile wide, making it a sublime day trip.
It’s the perfect place to have a picnic and a swim. Make sure to bring snacks and drinks, as there’s only one café on the island.