The Thüringer Schiefergebirge, known as the Thuringian Highlands in English, is an area of the German federal state Thuringia (German: Thüringen) featuring steeply hilly areas on the border between Bavaria and Thuringia, through which the Saale River cuts. As the German name suggests, this area contains a large amount of slate rock. There are very few people here, so others come from the surrounding areas to enjoy their time in relatively well presevered nature.
The area boasts many hiking and walking trails, including along the Saale on both sides (as seen in the image above). You can also hike throughout the hills. A great time to go is the beginning of autumn; the tree leaves are beginning to turn many colours. October is usually a good time for this. In this area you can also see castles or castle ruins, some of which you can see from viewpoints with amazing views.
If you want to visit it, head to Saalburg-Ebersdorf on the River Saale. There are several trails that start and/or end there. This lets you leave your vehicle there (or use public transport) and head off into the surroundings.
Here are some more photos from the area: