I will be traveling between Frankfurt and Budapest the first week in October, possibly by car. What sights and/or cities are a definite must-see between these two cities. Thanks!
How many days do you have for this excursion? According to www.viamichelin.com, the fastest driving route takes at least 10 hours. If you wanted to get there quickest, you would fly; see www.skyscanner.com. Train is also pretty simple, with one connection at Munich or Vienna. Nurnberg is another significant tourist stop along the route. Before reaching Vienna, you could consider smaller town stops along the Danube Valley and/or Mauthausen Concentration Camp Memorial.
If you're not pressed for time, I would suggest Frankfurt itself, Munich, Salzburg, Bad Ischl, Linz, and Vienna. Be advised that you can do this entire route at night, the EC night train runs from Frankfurt to Budapest. I took it as far as to Vienna arriving shortly after 0900 hrs.
If you are doing other than the night train and if you are stopping in or going through Vienna and if you are not in a big hurry to get to Budapest you might look at stopping for the night at Gyor. Gyor lies almost exactly halfway between Budapest and Vienna on the tracks / highway that connect the two cities. In Gyor you can see the old town center which is primarily baroque with a few medeavil reminders still extant. The Synagogue is especially interesting as are the river fronts and the shops and streets. The Hotel Klastrom is a converted 18th century cloister right in the center of it all. About 20 minutes away by car / taxi or guide is Archabbey of Pannonhalma and the Vineyards and Winery associated with it.