Mljet is one of the most appealing day trips from Dubrovnik, though it deserves more time if you can spare it. The western end of the island is Mljet National Park, built around two interconnected saltwater lakes — the larger Veliko Jezero and the smaller, warmer Malo Jezero — with a Benedictine monastery on an islet in the middle of the Great Lake.
Most visitors swim and cycle around the lakes — an 11km circuit with bike rental in Polače and Pomena — or ride the current through the Soline Channel, where the tide reverses twice a day. The village of Soline is the quietest base for exploring the park. Elsewhere on the island, the Odysseus Cave on the rocky southern coast is a memorable swim-through sea cave, its water turning bright blue around midday.
Ferries from Dubrovnik dock at Sobra, Mljet's principal port on the less-visited eastern end of the island; from there it's about 20km by road to the national park.