The Marché des Enfants Rouges, in the heart of the Marais, is the oldest covered market in Paris — operating since 1615. It is small by market standards but punches above its weight for eating: a dozen stalls selling Moroccan couscous, Japanese bento boxes, Italian pasta, Lebanese mezze, and traditional French charcuterie, with communal tables in the centre.
Arrive at midday on a weekday and join the mix of local office workers, Marais residents, and tourists who have found this place. The Moroccan stall in particular has a devoted following. The organic vegetable stalls in the rear are good for browsing even if you're not eating.
This is the best informal lunch in the Marais and one of the most convivial eating experiences in Paris. Saturday morning is lively and crowded; Sunday morning is pleasant and slightly calmer. Either way, arrive hungry.