Myself and two other older (55-60) guys will be driving from Amsterdam to Munich via the Black Forest in late September. We will be stopping over for one night enroute. We were thinking of somewhere around Strasbourg/Baden Baden/Oftenburg. Two of us will be celebrating our birthday that evening so we would like an authentic experience with great ambiance were the food is great and there are a number of establishments open until at least 11:00 pm. We've heard that some of the smaller towns close up early...
I have not experienced Offenburg but of the others you mention Bade-Baden for sure would suit your req.
There are many options and many price points and a great central area. And guys def do the spa thing in Baden -Baden, it is not fou- fou but healthy invigorating steam, dunk, massage type.
How about Volksfest in Stuttgart? Lots of authentic ambiance. You can get great food in downtown Stuttgart with lots of restaurants open until at least 11:00.