One of the nice things in Germany is the almost total accessibility of nature. You can basically go anywhere you like. Take your car along a small road, park it on the curb and start walking along a field or into a hilly wood, for as long as you want.
This is somewhat different from other countries like England, where private property, high walls and fences often stop you from roaming freely and limit you to a few public footpaths and of course to the wonderful coastal path. The coast-line in Germany is nowhere near as long and as pleasant as this, but there still are some nice walks along the Baltic Sea.
And then there are the many lakes, some in the North and some in the far South, most markedly the large Bodensee and the Chiemsee, with the wonderful Alps nearby. Very inviting are the big rivers like Rhein, Main, Mosel and Donau, always with a path close to the bank for a day-long walk or a bicycle tour, often leading you from one historic village to the next.
The only thing to remember is where you parked your car.