It is a more or less personal opinion, but I personally would first travel along the Rhine valley. There you can find nice landscape and also stop in beautiful cities like Strasbourg. After that I would carry on through Alsace, crossing nice cities like Colmar. After that I would try to cross the Alps, to see some high mountains and beautiful landscape. It depends a little bit on where exactly in the South of France you want to travel (i.e. to the border with Spain or Italy or between), but assuming that it is somewhere in the middle like Marseille, I would then visit the Swiss city Basel and travel along Lake Neuchatel and Lake Geneva back to France. There I would aim to Chambéry, located directly between two Natural Parks. And from there you can pass Col de l'Iseran, the highest pass in the Alps (2770m) to get to Italy. From there I would aim at Albenga so that you can later follow the beautiful Italian respectively France coastline to your destination.
Here is a map of my suggested route:
Google Maps tells me it is 1495 km, while the direct route would be 866 km. So you see it is really a lot more to drive, but you will also see a lot more!