As the title says, just looking to break up the journey between Heidleberg and Strasbourg. This will be via train or bus. What cities would you recommend (or not recommend) in between these two points?
Heidelberg to Strasbourg is only 2 hours. And there aren't really any places in between that are more interesting than Heidelberg and Strasbourg themselves. I would just spend a day more in either of them and just go straight between them.
In Germany, good options include either Baden-Baden or one of the small towns on the edge of the Black Forest, such as Gernsbach or (a little south of Strasbourg) Gengenbach. In France, consider one of the smaller towns in Alsace, such as Wissembourg or Haguenau.