Nice relaxing places between berlin and amsterdam

I'm travelling between Berlin and Amsterdam in a couple of weeks and I want to find a nice quiet, relaxing place along the way to spend 2-3 days. Somewhere with lots of nature around and walking trails, would be a bonus if its on a lake that I can swim in.

Any suggestions?

http://erlebnisregion-edersee.de/en/home/, this might work, just enjoyed a long weekend there.

Fritzlar is not to far from there and I thought it's a pretty little town http://en.wikipedia.org/wiki/Fritzlar.

You can't go wrong with #1's suggestion.
A bit more direct between Amsterdam and Berlin, maybe some inspiration
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For swimming many options for smaller lakes, just google town and Badesee (swimming lake), for instance Osnabrück: http://www.mein-badesee.de/osnabrueck

BTW tempelton, your first link doesn't work (on my machine!)

Edited by: arizona

http://erlebnisregion-edersee.de/en/home/ thanks, hope this one works, guess the comma at the end is to much and was not intended.

not directly between berlin and amsterdam, but not "out of the way" either - switzerland. around, in and on the thunersee and brienzersee (around thun, interlaken and brienz) plenty of places to swim, walk, beautiful nature and much more.

Directly between Berlin and Amsterdam is Bielefeld. The city with approx. 360000 citizens is located at the Teutoburger Wald, a medium sized mountain range. In Bielefeld you will find the Sparrenburg (a small castle) and a pleasant old town with cafes, pubs, restaurants. The Teutoburger Wald is ideal for hiking and enjoying nature. Unfortunately there is no lake or river in and around Bielefeld which is worth to be mentioned.
To learn more about Bielefeld and its region have a look here:
http://www.bielefeld.de/en/ti1/
http://www.teutoburgerwald.de/holiday/