Hi
I like your question, but it varies wildly where you are going. Agree with the packed lunches, pop into boulengiere for fresh bread and pastries, then supermarket or markets for a supply of pate, cheese, jambon and away you go. I usually take a small fold down cool bag (that fits into a backpack) with me. You can also buy a cheap knife etc. in the supermarket and away you go. The French love to picnic...
We regularly travel to France, latest trip went to South of France last Aug for 2 weeks. First to Marseille and then to St Remy in Provence.
We mainly go self catering but mix it up with cooking for ourselves, buying from weekly markets (which can be expensive or maybe it's just we get carried away) and big supermarkets e.g. carrefour and eating out. I did notice it was more expensive to eat out last year than the previous year due to the Euro.
When eating out order the "plat de jour" usually one, two or more courses and good value. For the kids we tend to order a few starters for them to share rather than an adults meal if no kids menu. We tend to have either a big lunch and then bread, pate, cheeses, ham, olives etc for a light tea or the other way around.
Also agree with taking tea bags with you if a tea drinker.
Hope you have a lovely holiday.