Potsdam

Potsdam is a city southwest of Berlin, distinct from Berlin, but included in the same public transit system, so reasonably close. The city is covered in green space, and hidden throughout are tons of churches, Prussian palaces, and all sorts of cool stuff. That being said, my favorite thing about the city was that it is intensely quiet and slow-paced. The only people who seemed to need to be anywhere were me and all the other tourists (typically much older than the average Berlin tourist, by the way). Here's a small sample of the sights in Potsdam:

Alt Rathaus (Old Town Hall)

Potsdam Bells

Belvedere auf dem Pfingstberg

Schloss Sanssouci

Schloss Cecilienhof - site of the 1945 Potsdam conference