I would prefer to hear of places that are not too dressy and are reasonably priced.

topper

TTT

One of my favorite spots in Paris is the Place de Contrescarpe in the Latin Quarter. It's surrounded by several modest cafes where you can get sandwiches, carpaccio, etc., with drinks, and they're a great place to people watch. I'd avoid the restaurants just off the square on the upper half of rue Moufetarde. I havne't tried them, but they look like tourist magnets. The lower half of rue Moufetarde has an entirely different character; it's one of the oldest and nicest street markets in Paris.

La Carbonara on Campo Di Fiori was very pleasant. Campo Di Fiori, of course, is itself kind of "funkier" than Piazza Navona (just three blocks north), and I found the people to be friendlier, the food to be better, and the prices lower!