Cat Ba National Park covers about half of the island and protects a mosaic of limestone karst, tropical evergreen forest, mangroves, and offshore reefs. The headline species is the critically endangered golden-headed Cat Ba langur, one of the world's rarest primates — confined to the far north of the island and almost never seen by visitors — but the park is also home to civets, macaques, and 70-plus bird species.
The most popular activity is the Ngu Lam Peak climb, a steep four-kilometre out-and-back hike from the park headquarters that takes two to three hours and rewards you with a 360-degree view over the karst peaks of the interior. A longer eight-kilometre traverse from the headquarters out to the village of Viet Hai is excellent: you finish at a tiny back-country settlement and take a boat back to Cat Ba town. Bring water, proper shoes, and a guide for the traverse; the path is faint in places and unmarked.