Luxembourg is out of your way, and you would have the thrill that is the Autoroute between Metz and Luxembourg via Thionville and Yutz. Lots of trucks, always delays. Personal experience. Luxembourg is a place I like quite a lot, both rural and the city of the same name. A lot of people aren't so keen, so its up to you. It will add an hour northbound and an hour southbound. Try to arrive in Luxembourg (the country) with an empty tank. Fuel, especially diesel, is the cheapest in western and central Europe. All filling stations in the country are the same price, plus or minus 2 tenths of a cent.
Nancy could be on your way, Karlsruhe could be on your way. Stuttgart is definately on your way, and the Mr Wash car wash there is the coolest I've ever used. I go back whenever there's a chance. There's even a youtube. I don't know what Sarah - who lives there - thinks of it. Just south of Stuttgart is Tübingen and east is Esslingen - both worth a visit if they float your boat.
With 8 or 9 hours driving you won't have a whole lot of time to smell the flowers.