First, is Basel a good day trip? if so, give me some options please ??
I liked Basel quite a bit, but everything is relative. They have a spectacular rathuis on a lovely marketplatz. The munster is interesting and there is a nice view of the Rhine behind the munster. We crossed over the Rhine and walked on the other side and the views were lovely.
Also close to Gengenbach is the Alsace area of France which I really love. Colmar is nice enough but I feel the real jewels of Alsace are the little towns like Eguisheim and Ribeauville. We followed the wine route and the views were quite lovely.
What time of year are you going?
Plan to be there mid September....Have seen much of the area (Alsace) and loved it.
I think you would have a lot of fun in Basel and in a day you could cover much of the touristic areas except for the museums. I contacted Basel TI ahead of time and they sent me some great maps and walking tours. Other day trips could be Frieburg, Breisach, Heidelburg, perhaps.