I've driven the Alps in winter to a number of ski resorts, so I can't really help you with good routes for the Summer. A number of friends have told me that the roads are quite busy in the alps during the summer.
I have driven the Massif Cantral during the Summer and winter though. I was there September 2 years ago. Best routes are around the area south of Lyon. I have one favourite route going from Valance to Le Puy (About 40miles south of St Etienne).
The roads are quiet. Loads of 3rd-4th twists and turns through vallies, under viaducts, through tunnels.
The last time I was there, we stopped at the top of Mount Mezanc and suddenly 2 dozen Ferraris arrived. The French Ferrari club must have decided to take the quiet Massif Central for they're trip aswell.
Must say that the sound of a Ferrari F40 screaming, popping and cracking through the vallies at full tilt could bring a tear to a grown mans eye!
Anyway, the Alps are probably worth it but the Massif Central is quiet and I'd be surprized if you saw any cops... Unlike the alps where I was ID'd everytime I went there(in a car). Fortunately I was wearing my get-out-of-jail-free-card (Ireland Rugby Jersey) the time I forgot my lecence!