This is a question which comes up pretty regularly. Did you do a search already?
Here's a thread to get you started:
Also, it would help if you mentioned your budget in €/pp and maybe the area you'll be in.
Personally, I like a nice terrace with brasserie food like La Rotonde in Montparnasse.
I searched "Sunday Paris" and didn't find any help in the first 20ish threads but now I see a bunch at the bottom of this page; thanks! I must have not been specific enough. ;) I'm looking for something casual, with good, unfussy fare, and good people-watching at less than 50€/pp. A good bistrot or brasserie would be perfect so La Rotonde looks promising! We're staying in St. Germain des Pres but are happy to travel.