I come from Estonia, which is in the north of Europe. Therefore, I can point out another extremely pleasant thing about Harz. Although it is mostly hills and forest, there are almost no mosquitoes or other insects in the summer, which would attack or annoy man. On the other hand there are a lot of "friendly" insects, which the local birds eat. It is such a relief for a northener to enjoy hiking in forest, or reading a book in nature in the summer without protective clothes, repellents, or constant hand waving. Even ticks of the Harz, which I have not seen yet, do not carry any disease. There are ticks in Northern Germany and also in Bayern, which are dangerous. Until now Harz is safe, but this is not guaranteed.
My personal favourite place in Harz is Stolberg/Harz, which is a small city, consisting mostly of medieval buildings with a beautiful Schloss on hilltop. The town is situated between forested mountains and is surrounded by many wonderful hiking trails and view points. There is also a nice spa and sauna centre with swimming-pool, open round the year. I am playing with the idea to retire there. After the beautiful Christmas time in Germany, I would fly to South-East Asia until april with my wife and then return to Harz to enjoy the spring and birdsong. Maybe one day I will make this happen.