The Marché d'Aligre is the best open-air market in Paris — loud, crowded, and genuinely used by the neighbourhood rather than curated for tourists. The outdoor section, in Place d'Aligre, sells vegetables, fruit, and cheap second-hand goods. The covered Marché Beauvau alongside it has excellent cheese, charcuterie, and prepared foods.
Come on a Saturday morning. Eat oysters at one of the stands around the square. Buy a bottle of wine from the cave around the corner and have a genuinely Parisian morning for almost no money. In the 12th arrondissement, near Ledru-Rollin Métro.