Most national capitals have decent flea markets and antique districts. Paris is no exception, and you can find both types of markets nearly side by side at the northern edge of the city. Take the métro to Porte de Clignancourt or Porte de Saint-Ouen, on or near the Blouvard Ney, and follow the people. Look for lots of awnings, old trucks, immigrants/tourists, and junk. I went to shoot the flea market, but found the antique district much more interesting.
It was Saturday, so cafés were busy on that early October fall afternoon.
Being near the Périférique (highway which encircles Paris), graffiti was everywhere.