We're here too.
Usually when we have friends visit we take them quickly through innenstadt then out to the mountains (Taunus) and villages there. Großer Feldberg is accessible by public transit, I believe. Not the most exciting city we've lived in...
Along Wallstraße in Sachsenhausen there are some traditional-style restaurants, we usually go to Fichtekränzi.
There is a René Magritte exhibition at the Schirn. Quite a few museums along the river in Sachsenhausen too.
People seem to like Heidelberg. I don't know if the train to Stuttgart would take you through it. To the east Würzburg is a nice place to visit, if you want to travel a bit.
By car, the Ring is easy 2 hours each way, and you can take the scenic route along the Rhein.
I would offer you a tour of my work but the racecars are on the way to Portugal at the moment so there is not much to see actually.