The island's highest point, reached via a trail through Cat Ba National Park — a genuinely sweaty climb rewarded with a full view over the limestone karsts scattered across the bay.
A limestone cave system inside the national park with stalactite formations still forming — a shorter, easier outing than the peak trek if time is tight.
An overnight (or multi-day) boat trip through the bay's thousands of limestone karsts is the classic way to experience it — Cat Ba functions as a genuinely cheaper, less crowded departure point for this than Ha Long City on the mainland. The bay's name comes from a legend of a descending dragon whose thrashing tail carved out the karsts and inlets as it dropped into the sea.