In the same vein, if you don't want to deal with the prep or carry around/throw away extra food, around lunchtime at most boulangeries you can get a generously-sized baguette sandwich for about 3 euros, or often a formule that includes a sandwich, pastry, and beverage for 5-6 euros.
Gotta agree with the poster here - having a baguette sandwich for lunch is quick, cheap, and can save a lot of time vs. sitting down for a meal. There are shops selling sandwiches (and lots of other yummy things - like French pastries!!!) everywhere. Grab one and go! Makes me want one right now!
Another great option if you like them (and who doesn't?) are crepes. Many of the same shops that have baguettes have crepes of various varieties, too. The place right across from the entrance to the Notre Dame tower climb (
Les Tours de Notre Dame) has great banana and Nutella crepes as well as good sandwiches. I have the picture of my daughter eating her crepe to prove it! Enjoy!