My husband has business in Paris and Frankfurt in June. We are thinking of driving between the 2 cities so that we can see some of the countryside in between. What would be our best options for sightseeing as well as enjoying some downtime in quaint French and German towns? We have allotted 3-4 days for the trip but that's not set in stone.
You could visit Reims and the Champagne region, Verdun and the battle fields of WW1 and then on the German side either visit Speyer, Heidelberg, Schwetzingen (castle, gardens) or from Saarbrücken follow the Saar river northbound (plenty of quaint villages, towns, castles) towards Trier and then visit the Mosel river valley. From Koblenz follow the Rhine river southbound towards Mainz from where it's only a 30mins drive to Frankfurt.
Just as an idea, we are doing the reverse trip and are planning to take the high speed train from Mannheim (just sounth of Frankfurt) to Paris. Unless you particularly want to drive, and you certainly could stop in little towns along the way if you did so, you could take the speed train (4 hours) and "see" the scenery and then rent a car from Mannheim train station and drive around Germany, especially the Rhine area, which is just north of Mannheim, on your way to Frankfurt. My undeerstanding is that it is about a 1 hour drive to the Rhine area, then 1 hour or 1 1/2 hours to Frankfrut from there. Have a nice vacation.