There are 60-70 street markets (and market streets) in Paris on various days of the week. My fave is the Marche d'Aligre in the 12th (Tues thru Sun, less touristy, more locals, a mix of flea market and food stalls and covered food hall), but you can find lists of many of the others online and in Rick's Paris book. If you're there late in the morning as they're starting to close up shop, have a wonderful/inexpensive Moroccan tagine at one of my fave restos: La Ruche a Miel at 19 rue d'Aligre and end with hot mint tea.
The absolute closest street market to you is on Blvd. St. Germain at the Maubert-Mutualite Metro stop on Tues/Thurs/Sat (the caramels and caramel sauce are great!). There are also several other shops right there, including Laurent DeBois for cheese, etc. If you then walk a few blocks up rue Monge (away from the river), you'll find traditional sweets (free tastings!) from all around France at Le Bon Bon au Palais at 19 rue Monge (near cross street rue de Ecoles). As noted, there are also markets at Plc Monge and at the far end of rue Mouffetard.
Have fun (and know that we're all envying you)! :D