Potsdam : Easy accessible from Berlin with regional train or Tramway.
Potsdam was until 1989 (fall of the Berlin Wall) under the German Democratic Republic (communist) and you can feel it when you arrive in Potsdam : wide and long streets, big and high buildings. It looks like some places in Russia or Ukraine.
However, Potsdam has also the particularity to be german, and to have a historical (young!) center, with colored building, typical german (and sometimes bavarian) architecture. Also, there are offers to eat (currywurst mit Brot = curry sausage with bred) or you can go to the market if you prefer fructs and vegetables. It is also interesting to visit the center, and not only going to Schloss Sanssouci.
Schloss (Castle) Sanssouci ("without trouble" in french) was build for the king of Prussia Friedrich der Grosse. It was finished in 1747. The style is approaching Chateau de Versailles near Paris, but the architecture is a bit different (rococo also for Sanssouci), and it is much smaller than Versailles. But you will enjoy the guided tour (obligation) in your language! Also walk in the park, it is very big and you can enjoy other buildings and the quite, because there are much fewer tourists than in Versailles!