If you are visiting Germany during the beautiful days of Autumn or Herbst as it is called in German, you should definitely visit the Eltz Castle and the longest suspension bridge in Germany - Geierlay in the Mörsdorf region.
Eltz Castle is accessible by public transport but there are many points where you can hike to this hidden gem along the Mosel river. You can make this trip in a day if you are staying in any of the big cities like Frankfurt, Koblenz or Trier.
I have been to this castle thrice, all three seasons hiking twice, once from Moselkern and once from Treis-Karden train stations while once we just drove there by car. If you hike, the trails are clearly marked and most trails are easy to moderate levels. The castle never failed to splendidly present itself be it summer, winter or fall.