I'll second the motion for Casse-Crepe Breton. Wonderful Crepes. If possible, it's best to go in late summer or early fall when the front windows are open and you can sit and watch the street go by.
Also, not so much a food experience, more of an earthy nightclub, check out Chez Son Père. Located in the old city, it's a second floor cash-only bar featuring live folk music nightly. Lots of fun.
We had a nice light meal at Chez Temporal. Good Soup & Sandwich place.
A local steered us toward Cafe Brouard. Definitely nothing fancy, but a good standard diner type meal. It was comparable to what we here in Chicago call a "Greek Restaurant", meaning it is under Greek ownership, not necessarily serving Greek food. Wide-ranging menu, specializing in nothing and doing everything moderately well. i know I'm not selling it very well, but it's a place frequented by locals, and it's better than corporate fast food.
Be sure to hang out on the boardwalk after dark surrounding Le Château Frontenac. It's like a minor league training ground for Cirque du Soleil as hordes of street performers line the area. Sword swallowers, acrobats, fire eaters, and more all try to gather a crowd then give you a demonstration of their talents. Each one puts on a quick show lasting about 10-15 minutes. Drop a Loonie in the hat and move on to the next performer. It's a wonderful way to spend a balmy evening.
Hope that helps. Have a great trip!