There's always the Bon Marché (see Zola, Au Bonheur des Dames aka The Ladies' Delight for the story of the growth of a department store--or as in Babar "The Big Store" (le grand magasin)--in c19 Paris) of course, nice to walk around, but only on a day you leave your purse and your appetite at home. I go there if I want something normally cheap and useful, a very nice tea towel, say, the one you hang on the oven and never use, but which matches the floor tiles and the set of mugs you found at Monoprix.
Monoprix! I haven't been there yet. It's a treat I'm saving. Monoprix on the Bd St. Michel, in between two branches of the bookstore, Gibert: papeterie and three floors of books, one on the corner of the rue Racine (which meets the rue Corneille at the Odéon) and the other on the corner of the rue des Ecoles. Groceries downstairs, scarves and face creams and toothpaste on the main level, and upstairs, clothes and colour-coordinated household goods, mugs, plates. Everything here changes every month or so, including the colours, and it's a great place to pick up a bargain whatnot or other. A pair of slippers. It's on my way to and from the gym, behind the Musee de Cluny, whose courtyard I ducked into yesterday. Sure enough the hot pink hollyhock was poking up through the cobbles, almost a weed, unplanned, but left to return again and again.