Almost everything worth doing in Halong City involves getting out onto the water. The bay itself is the draw, and the best access is by cruise — overnight or multi-day trips that take you away from the tourist concentration around Bai Chay and into the less-visited corners of the 1,500-square-kilometre bay. On land, the city has the Quang Ninh Museum (striking architecture, reasonable collections) and a coal mining heritage worth understanding if you're curious how this part of Vietnam actually works.