It's hard to get a meal outside of the traditional mealtimes (12-2, 8-10:30), but cafes and brasseries that advertise "service contenu" will serve outside of mealtimes. Sometimes this may just be light fare like salads, sandwiches, omelets, but not every meal needs to be a fancy three-course meal.

Chartier is about the ambiance, and they do open at 6 IIRC. Cafe du Commerce, across town, does have better food. There are other big, old-fashioned cantines of similar description with prettier decor, like Bouillon Racine and Julien. Prices will probably be higher but the experience more fun/eye-popping.

I second the suggestion to try a creperie. The atmosphere tends to be informal.