I used to visit most of the markets in Paris on a rotating basis, because I was younger and had a lot more energy and time - but it's too difficult now, especially in the Summer. Plus, I'm too old to haul many kilos of fruits and vegetables from a market across town - and I don't care anymore whose famous garden they sprouted from or who else shops there. Now, I stick to Bastille and sometimes Aligre, if it's not too hot outside.
This morning, I got ready to go to the Bastille market, and instead of putting on my usual anti-tour-group battle gear, I just put on a straw hat and a smile. In fact, I had a pretty good time - though you might not find the experience as enjoyable as I did, so here's some advice about how this market changes in the Summer.
At Bastille, a significant number of vendors disappear - going off to sell things in other parts of Paris or France or even farther away than that. Only about half of them remain, but there's still a wide variety of quality and prices, and still a good selection of food items. This market is open Thursdays and Sundays (about 9 - 12:30), and Thursday is always less interesting - and less crowded. But in Summer, the Bastille market undergoes a more radical change, and this is really obvious on Thursday.
The mainly-food market becomes a "souk" - emphasis changes to cheap, colorful clothing (lots of it is the kind of thing that falls apart after one washing, so don't get overly-excited), various accessories, "Ron-Popeil-type" gadgets, and so forth - and the collective atmosphere changes, too. I have brought friends and family here in the Summer, and they were really disappointed, because it wasn't at all what they expected. I saw lots of sad tourists walking around today, too.
Vendors are more insistent - not aggressive, but they really-really-REALLY want you to buy something - and at the same time, they will take time to talk and joke with the customers, too. Usually, the place is much too hectic to stand around discussing a plant you just bought with the lady who's selling you far more cherries than you could possibly eat in a month - but I enjoyed that, today. My French got a good workout - usually during the year, vendors size you up at a glance and start speaking the language they think you do - even if they're wrong - because it would be quicker for them to make a sale. This time, when I spoke French, nobody commented on my "charming accent" - they just responded in French, which was really nice. Usually, someone tries speaking to me in Polish...
So, after about one hour in the hot sun, I came home with a cart full of watermelon, charentais melon, the last good cherries of the season, peaches, prune plums (which I should be eating right now, instead of typing - they are that ripe!), shrimp, avocados, one potato and a nice, red Dipladenia to fill a spot in my window that will help me avoid looking at my grumpy neighbor across the street. I was tempted to buy a brightly-patterned shirt, but knew I would never wear it in public, and was afraid I might frighten my puppy if I wore it at home.
So, to wrap things up - if you're in Paris in the Summer, just be aware that the Bastille market will not be "all that". Sunday will be a little more interesting than Thursday, and this Summer mode lasts from mid-July through mid-August. Lower your expectations, take some nice photos if you like, and buy much less of that beautiful food than you want to. There is more "street food" available on Sundays than on Thursdays, and the nearby cafes are open, if you want to sit and relax over a cool drink.
It's important to know that during this time of year, things ripen much faster than you can possibly eat them. Instead of buying "un kilo" of cherries (2.2 lbs), you can ask for "une livre" (oon lee vruh - 500 grams, or about 1 lb), or "une poignee" (oon pwahn yay - a "handful", 200 - 300 grams, depending on the hand) . When those are gone, you can buy some more elsewhere, but at least you won't have wasted money or thrown away food that went bad before you could enjoy it. In fact, I think those prune plums will have to be made into compote, in about five more minutes...