Mary, a good day can be had if you leave early and go to the open air museum, catch Gengenbach on the way back.
"I am assuming no hiking would be easily accessed." - Hiking is easily accessed most places in Germany. If you enjoy a long hike, the museum can be left for another time. There is an easy walking path by the Kinzig river in Gengenbach. For more hiking in and around Gengenbach: https://www.gengenbach.info/gengenbach-entdecken/aktiv-fit/wandern
If you have more than one day to spare in the Black Forest, Baden-Baden is about an hour away. Baden-Baden gets a bit of flak on this forum but I enjoyed my last visit there a few years ago, despite it being teeming with Russians. There are spas, ritzy stores, trails, a lovely stretch of a garden, and nice residential areas.
A third day to spare? I haven't done this but there's a scenic train ride from Offenburg to Freiburg, via a change at Donaueschingen.
Slightly off topic but try and make it to Wissembourg in Alsace. It's an attractive and little visited village, slightly out of range of the usual Alsace itinerary.