This crazy market takes place every Sunday, from early in the morning until early in the afternoon, next to Gare du Midi. It is huge, it is crowded and it is fun! You'll see all kinds of stuff being sold there: from clothing to shoes, fruits&vegetables, kitchen accessories, rugs, food, bags, you name it. It is all very chaotic, but nice to see this animated market atmosphere and you can save some money on good deals.
The best place to eat when the hunger comes after all the strolling around is this big olives' stand. Here they don't only sell all types of olives, but on the back they also make this delicious pancake with cheese, olives and honey for just 2,50 euros. I cannot give you precise indications on where to find it, giving the big area where the market is spread and the lack of reference points, but just stroll around and look for an olives' stand with a big queue of people on the back, there you'll have found it! But don't worry, because the big queue moves fast and you'll be able to taste your desired pancake very soon.
For the lazy mornings ones, I was told this market finished at 14h, but I left at 13h30 and it was still alive and kicking, full of people and with all the stands on, so I suppose it will last during part of the afternoon as well. Enjoy!