Here is a
nice itinerary from Munich to Frankfurt, which should give you plenty to see and do:
i- You start in Munich, which has a nice christmas market and lots of great museums. But you need to tell us in what direction you want to go (arts, science, ...) with the museums.
- You drive to Regensburg, which has a really nice old town and a nice christmas market.
- Then you go to Nuremberg, which has the most famous christmas market in Germany. In terms of museums, I would recommend the "Germanisches Nationalmuseum" (Germanic national museum).
- Then try Bamberg, which has lots of good beer, is one of the few cities that didn't get bombed during the WW2 and is definitely worth a visit.
- On your way to Würzburg (which has a christmas market, too), where you should visit the residence and the castle, please make a stop in Königsberg, which has a really nice old town with lots of half-timbered buildings, but is not nearly as touristy as the likes of Ruedesheim or Rothenburg ob der Tauber, that you hear a lot from.
- Then in Frankfurt, try the christmas market and then head over to the Museum of Modern Art (5 minute walk from the main christmas market) or head over the Main river to the museum riverside, which offers lots of great options. I personally like the Staedel museum (lots of classic and modern art), the Liebighaus (sculpture museum) and the German Architecture museum.
I would recommend staying in Munich, Nuremberg, Bamberg and Frankfurt, but the route is flexible and you can leave stops out or just stay a few hours here and there and then drive on.