Not quite sure what you mean by Strasbourg area … is that just Alsace? What is the draw for you? History, colorful villages, food, wine, churches, other?
We spent six nights in Alsace in September. I don’t know that I’d choose more than about eight nights in the Strasbourg to Colmar - Route de Vins area of Alsace, personally. We did it by e-bike with two nights in Strasbourg, single nights in Obernai and Ribeauville and two nights in Colmar. We could have enjoyed one more day in Strasbourg and Colmar each to see more in those two small to medium size cities. The many lovely towns of colorful half timbered buildings, small churches and family wineries throughout the area are charming but they do not required days on end to enjoy. Three days to visit or stay in about nine different villages (plus two castles) was ample.
The 1000 year old Notre Dame de Strasbourg is magnificent and its history and its ‘chantier’ - its having been a work in progress for a millennium - is compelling.
We missed the Musee Unterlinden and Musee Bartholdi in Colmar. The former because we had seen plenty of altarpieces already in Strasbourg and in other churches in Kaysersberg, Burgheim and elsewhere and the latter because the museum itself was closed the day we went there to hear a concert in its courtyard as part of Les Journees du Patrimoine - Heritage Days.
Mardee suggests Gengenbach and the Black Forest. We spent a couple of nights in Freiburg plus another night in the Black Forest near Schiltach in advance of our visit to Alsace and we then spent five nights in Burgundy after Alsace. Also nearby is Lorraine and then the Champagne region and the WW1 battlefields such as Verdun.
Any one of those four areas could be a nice complement to Alsace … whether wine, history or food is near the heart of your journey. (Our journey did not include Lorraine, WW1 battlegrounds or Champagne; you can never do it all.)
Whether you need a car for your 12 days is questionable, but a car can help for limited parts. We had a car for just five of our 30 days in SW Germany, Eastern France and Paris. We took the train from Freiburg to Strasbourg at the front end of our Alsace segment and then travelled by train from Colmar to Dijon at the back end. Otherwise all travel in Alsace was by e-bike , including the climb to Haut Koenigsburg in the way from Obernai to Ribeauville.
(Late Note - Our Alsace/Burgundy TR is here)
In your case, perhaps you might choose a few nights in Strasbourg and Colmar plus another two nights in-between and just use a car starting with a departure from Strasbourg and ending upon arrival in Colmar.
Bon Voyage.