A fun thing to do on a Saturday or Sunday morning is to head to the Marché aux puces de la Porte de Vanves (Vanves Flea Market), between avenue Marc Sangnier and avenue Georges Lafenestre. It usually has around 350 merchants, so it fairly easy to navigate in a few hours. The prices are reasonable and there is a nice selection of smaller items that are easy to pack and take back home. (Note: most merchants take cash only).
You never know what you are going to find at Vanves. One time, I came home with some pieces of vintage wallpaper (that I still need to frame). Another time, I came home with a large burlap bag that was used to tote newspapers. I love to use the miniature champagne bucket that I found there as a flower pot. But probably my favorite find was an Art Deco style serving tray with lucite handles that I use to display some of my bar accessories.
The market is open from 7:00am to 2:00pm almost every Saturday and Sunday of the year. I had heard that you needed to get to the market very early to get the "good" stuff. But most times I arrived at the market around 10am and was able to find quite a few treasures. I took my children a few times and they also enjoyed the treasure hunt. During the summer months you can expect the market to be more crowded, so it might be a good idea to hit it early in the morning with kids.
It is in the 14th arrondissment, but I found it was an easy Metro ride on the #13 to the Porte de Vanves station. When you get off the Metro, take Sortie #2 (Exit #2) and then once you exit the station look for the signs to the market and/or Avenue Marc Sangnier.