Moderate restaurants & cafes, that I've enjoyed recently in Paris, would be good for dinner (or lunch) I think you will find something you like in them. All are charming and in great locations. You can undoubtedly view more info about them on Trip Advisor or their websites
Bistrot Vivienne, in the Gallerie Vivienne, north of the Palais Royale
Le Fumoir, across the street from the east end of the Louvre
Le Hangar, 1 Impasse Berthaud (in an alley across from the Pompidou)
Cafe Constant, and Cafe Cocottes 133, 135 rue St. Dominique 7th arr. (small, very popular, crowded at dinner)
Les Philosophes, 28 rue Vielle du Temple in the Marais
Au Relais, 48 rue Lamark, down the hill in Montmartre
Le Rostand, across from Luxembourg garden (where Medici fountain is)
Laudree, Champs Elysees, and other locations
casual dining, lunch or dinner:
Coffee Parisien, 4 rue Princess, 6th arr. and 2 other locations. (American diner food.)
L' as du Falafel, Rue des Rosiers, Marais (usually a line out front, but it's very well managed and I've never had to wait long for a table, very friendly, interesting scene)
Lizard Lounge, 18 rue Borg Tibourg, Marais (sandwiches, burgers, happy hour - run by Americans)
Le Nemours, Place Colette. (good place to sit and people watch, I like their cassoulet)
Cafe Hugo, Place des Vosges