One of my favorite places to get a quick bite to eat when I was living in Paris was L'as du Fallafel. When I was there last month visiting, I decided to grab a sandwich but I picked the wrong day- it was the half marathon and I literally couldn't cross the street. I went back to the apartment and ventured out after the race was over.
It closes Friday afternoon for Shabbat, reopening on Sunday, which makes it a perfect pit stop for Sunday lunch. Much of the Marais in fact, is the place to be in Paris on Sunday, as many other parts of the city (shops, bakeries, etc.) shut down. There is often a line but it moves quickly as you pay while you're queuing. While they have more to offer than just falafel, I recommend the falafel sandwich with all the fillings (cabbage, cucumber, onion, fried eggplant, creamy white sauce, etc.) and sauce piquante (though it's not at all spicy). It all comes together nicely in a messy street-food kind of way. Take it to go and enjoy it (start with a fork, then eat it like a sandwich) in the street walking through the Marais or find a nearby park bench to savor it.
L'as du Fallafel is located at 34 Rue des Rosiers in Paris