today i just drove to random lakes in forrest regions. one had a free public outdoor gym (with 3 machines) and playground with 2 hammocks and a soccer field and picnic tables.
on the other side of the lake there was a beach, 4€ admission and 2€ parking.
i wouldn't know how to decide on a place, that sounds like a nightmare. having to rely on this marketing infested tourism industry.
I actually do remember a place i can recommend:
https://www.openstreetmap.org/search?query=%2067435%20Neustadt%20an%20der%20Weinstra%C3%9Fe#map=13/49.3497/8.1980
i was in this place called Neustadt an der Weinstraße and then by coincidence i drove by this Weinstraße that this place has in the name and it was breathtakingly beautiful. curves and hills and valleys (reminded me of a rollercoaster) filled with grapes that were growing for wine production, little wine tasting houses that looked like hobbit houses. reminded me of the windows xp default desktop background.