If you find yourself in Montevideo, Uruguay on any Sunday during the year, you should plan to pay a visit to the Tristan Narvaja Market for a manageable bit of sensory overload. As with all outdoor markets in Uruguay, it's referred to as a "feria" rather than a "mercado" and it's named after Street/Calle Tristan Narvaja, which forms the central part of the market. In reality, it's a huge flea market offering everything from food to fantasy. You should plan to spend between one and two hours visiting this feria located in the central part of the city (neighborhood Cordón) relatively close to the old city, or Ciudad Vieja.
Things to look for include used and antique books, decorative items (especially Art Deco and Mid-Century Modern), handmade jewelry and craft items. There’s generally not much room for negotiation from the quoted price, but it’s worth a polite try and more likely you purchase more than one item.
As with any crowded and active venue, be aware of your possessions, and keep track of your travelling companions... It's not that difficult to get lost in a market that covers 10 city blocks.