Beihai sits on Guangxi's southern coast, facing the Gulf of Tonkin, and it is the province's only significant beach destination. The city has grown fast and somewhat chaotically, but the seafood is outstanding — fresh, cheap, and available in quantity at the night markets, where the combination of subtropical warmth and cheap cold beer makes for a genuinely good evening. Silver Beach, the main draw, has fine pale quartz sand and on clear nights produces bioluminescence in the water, which is the kind of thing that sounds like a tourist-brochure invention until you see it.
The colonial-era Old Street is a pleasant few blocks of arcaded shophouses from the late nineteenth century, now given over to craft vendors and small bars. Beihai has a small but interesting aquarium with a genuine collection rather than the cash-machine approach of most Chinese waterfront attractions, and the Jinhaiwan mangrove forest on the edge of the city covers a surprisingly large area and repays a couple of hours of slow walking through its elevated boardwalks.
The real reason to make the trip from Beihai is Weizhou Island, accessible by ferry in about an hour. Weizhou is China's largest volcanic island in the South China Sea, ringed by coral reefs that are among the healthiest in the country, and two nineteenth-century Catholic churches built by the French sit surreally amid the basalt fields and fishing villages. The diving is good. Snorkelling is good. The island is quiet in a way that the mainland coast is not.
Beihai is 1.5 hours from Nanning by high-speed rail, making it feasible as a one-night detour. It works best as a wind-down stop at the end of a Guangxi trip, when you have earned a day of sitting by the water eating things pulled from the sea.