I did a lot of mountain biking when I was on holiday in the South of France this summer, in an area called La Provence Verte. Routes there have been approved by la Féderation Française de Cyclotourisme (FFCT). They have 21 different routes, some easy, some a lot more difficult, and you can hire mountain bikes when you get there. I had a fantastic time! The scenery is amazing, from the beautiful Mont Sainte Victoire and the Sainte Baume Massif to Lake Carcès. On the tourist office's website you can download some of the routes!

http://www.la-provence-verte.net

Click on English, then documentation, then tourist guides.