In reply to dannycee:
As others have said: Cantal, the area around Mont Dore (Massif du Sancy). I know a few possible spots for wild camping around Mont Dore and Super Besse - in my experience the Super Besse side of Sancy is a little quieter.
Also worth considering is the Vosges area in the NE. Easily accessible from Strasbourg and some good hills to explore, huge forested areas and definite possibilities for wild-camping if discrete.
For this kind of thing, though, the quieter areas of the Pyrenees and its foothills would probably take some beating. Around the Pic Midi d'Ossau and environs should give you plenty of possibilities. Although it can get busy in summer in some areas, it is easy to escape the crowds.
Post edited at 16:14