Foto: Librarian Avenger, flickrOne thing that's outstanding in Paris is the daily markets, which closely resemble the farmers markets known from the US. There are markets all over the city on streets and plazas. The markets have set locations and days where they're open. In the suburb where I live the market is in front of the town hall and is open every Sunday, Wednesday and Friday morning. The market starts during mid morning and is usually closed around 1-2 pm.
The mood is truly unique at the markets. You'll hear vendors shout their offers to the audience, people chat and have a great time, examine the wares and enjoys the good weather. Scents float through the air and especially the fried chickens on spears will make your stomach growl. You buy them cooked and bring them home in a special bag with juice and potatoes. That's the Sunday lunch. Bad weather doesn't keep the French away from the markets, they're still an important part of the culture.
You can find a market every day in Paris, it's just a matter of knowing where to look for one. Every arrondissement has at least one market. There are over 75 markets in Paris proper, so they're not hard to locate. Every suburb will have at least one regular market and the same is true for any town with self-respect.
The French still shop a lot at the markets, both for groceries, but also clothes, furniture and various stuff. Unlike in Denmark you can trust the vendors at the French markets and find some really good offers. It's certainly not "Hjallerup Marked" (a Danish yearly market known for its horse vendors selling horses under horrible conditions). The weekly markets still compete with the supermarkets and the ordinary shops in France.
Bring cash, it's easier and at many vendors you can't use cards.
List of markets in Paris with dates and times: http://www.paris-france.org/en/Living/markets/markets.ASP