Which scenic road to take to the south of France?

Take the "route des vaccances" (Nationale 7). It is a mythical road in France and crossing fine places, such as Bourgogne, Auvergne and the Rhône Valley. No "autoroute".

Officially this road starts in Paris. But you can avoid it. Travel through the Ardennes to Reims (via Charleville-Mézières) and then to Fontainebleau, where you rejoin the N7. If you come from one of the Luxembourgs, one of the Limburgs or Liège, you can easily reach Charleville-Mézières via Libramont, Bouillon and Sedan, as shown on this sample itinerary.

This first part to Fontainebleau involves some portions of "autoroute".

If you love moutains, there is an alternative road that runs through the Ardennes, along the Moselle, across the Vosges, Franche-Comté and Jura to Grenoble, from where you can take the scenic Route Napoléon (N85) to Grasse. Lots of fines places too. There are some small portions of "autoroute" involved (Thionville - Nancy and Chambéry - Grenoble), but it is straightforward to avoid them if necessary. Or you can add more "autoroute" if you want to shorten a bit.

This road can be done in three days if you come from Belgium, Luxembourg or Dutch Limburg. If you live further North, it can be wise to foresee one additional day.