The Villa Biarritz Flea Market is an open market, with hundreds of local small vendors. They sell clothes, artisan goods and some food. Prices are good and the quality of the clothes are OK. It is a common Saturday morning trip for a lot of people.
If you need a souvenir, a t-shirt saying "Uruguay" or a good cost/benefit in clothing you need to go there. For sure you will find a good deal. There is also a growing number of antique vendors and artisan goods.
Eat in the center of the square, ask for a "chorizo al pan con todo".