Although I've heard this is changing a bit, most French do not take their lunch to work. In French cities and towns there is bistro or café near where one works, and two hour lunches make it easier to go out for le déjeuner. But here in America, I teach school and take my lunch to work, as do many of us. But that doesn't mean I can't enjoy a delicious déjeuner.
For summer lunches in France, we almost always have salade composée: a handful of mixed greens and to which we add hardboiled egg, beans, tomato, olives--whatever we have on hand. For lunch salads we'll often add bits left over from last night's dinner--haricots verts, roasted potatoes, even chicken or beef--all tossed with a vinaigrette. The salade composée makes an informal but lovely presentation, and when paired with bread (to absorb the leftover vinaigrette), is a satisfying lunch.
This is something that I have continued to do here at home. After dinner each evening, I'll save any leftover bits to make a lovely lunch salad. In the morning, I add them to some baby greens--mesclun, romaine, mâche--put all in a lunch container (I use a small divided bento-type keeper from the Container Store), et voilà! At lunchtime, add a drizzle of olive oil and Alessi white balsamic vinegar for a delicious déjeuner. Simple, healthy, and very French.