The Antique Market at Chaotian Gong in central Nanjing is a great place to poke around and maybe stumble across some odd treasures. Although many vendors come to the market every day, it's biggest on Saturday mornings when many small collectors, dealers, and vendors come from the towns around Nanjing and even from neighboring provinces to display their goods.
The market is held in the forecourt of a Ming Dynasty Confucian temple - the carved entrance gate, red painted walls and ancient flagstones make a fitting setting for this fascinating market. While you might stumble on any sort of curiosity at the market, especially on Saturdays, some types of antiques are more common. Small carved jades, old woodcarvings, porcelain are easy to find, mixed among all sorts of other oddities and curios, ranging from old military medals to bronze incense burners to jade pipes. Of course, some of the items are fakes of varying degrees of quality, but it's still a fun place to poke around.
The Chaotian Gong temple itself is well worth visiting. A large complex of imperial buildings, it was constructed as a school where the children of local notables would learn the court rituals and Confucian ethics. While the historic buildings alone are interesting, today they house the Nanjing City Museum, with a collection of historic artifacts and exhibits explaining Nanjing's rich and many-layered history.