The Mercado Hidalgo is the city's main traditional market, spread across a large 19th-century iron-and-brick building and spilling into the surrounding streets. It is a working market first and a tourist attraction second, which is part of what makes it worth visiting: the produce, meat, and cheese sections are as busy as the craft stalls, and you get a sense of the city's actual daily life.
The stalls to seek out are those selling Santa María rebozos — the silk-and-cotton shawls woven so fine they can be passed through a wedding ring. These are a local specialty that has been produced in the nearby town of Santa María del Río for centuries. Also worth trying: queso de tuna, a dark sweet candy made from prickly pear cactus fruit pressed with sugar, and enchiladas potosinas, the local red-chile version of the corn tortilla dish. You can eat both at the market fondas on the upper level.