Although it covers 54 sq km of land and sea, including the entire westernmost quarter of the island, when most people talk of Mljet National Park they're referring to the small section that's ticketed, taking in the gorgeous saltwater lakes Malo Jezero (Little Lake) and Veliko Jezero (Big Lake). The two are connected to each by a short channel, while the larger one empties into the sea via the much longer Soline Channel, which makes the lakes subject to tidal flows.
The main hubs of the park are the small villages of Pomena and Polače, which are packed with visitors on summer days but quieten down again once all the boats leave. Kiosks in both villages sell park admission tickets. From Pomena it's a 400m walk along a forested path to Malo Jezero; from Polače, your ticket includes a transfer to Pristanište on Veliko Jezero, where there's a park information centre.