Saxonian Switzerland would be probably nice combined with Dresden old town.
Also Switzerland would be a good option (if your budget allows) - e.g. area around Interlaken or Valais 

Three travel days would support a day there, a day touring and a day back. 

Possibly base yourself in Munich and then have a day train trip to an alpine town (Many to choose from) e.g. Garmisch Partenkirchen for anything from a stroll to a full blooded hike.

Alternatively if you prefer history, from Munich you could visit Nuremberg, Regensburg and Amburg using ICE trains for quick transfers. Not nature or quiet , but well worth a visit.

In Belgium there are some really nice hikes from station to station, with good hiking guides. The nicest and quietest ones are in Wallonia, in the Ardennes.

Here some suggestions (most of it, is in Dutch, maar dat moet geen probleem zijn ):

https://www.caersbart.be/wandelen-treinstappers/#Overzicht_per_provincie-2

http://www.groenehalte.be/wandelingen/

https://www.belgiantrain.be/nl/leisure/b-excursions/more/green-stops/by-foot-green-stops