Make sure to go to Potsdam by taking the S-Bahn to the main train station. From there, buses take you up to Sanssouci Castle splendidly located in a beautiful park containing other beautiful castles. 
Sanssouci, intended to provide room for Friedrich, the "private man," nowadays invites everybody to discover beautiful vistas in abundance. After having taken the tour through the splendidly decorated rooms of the castle, stroll down the terraces designed to catch the sunlight that makes olive-trees and wine-stocks grow . . . . .
. . . . until you reach the central fountain and enjoy the view. 
Don't be surprised when you, strolling through the park, come across a "Chinese" Tee-Pavillion with every detail designed to fit the contemporary taste of Friedrich and his guests:the magic of the place will make you forget that you're so close to Germany's bustling capital.
When you have taken a rest in one of the countless shadowy places of Sanssouci Park, you might be ready to explore the Picture Gallery, The New Chambers, Charlottenhof Palace, Orangerie Palace, or step into the Church of Peace:
From this part of the Sanssouci Park, you can easily walk to Potsdam's "Brandenburger Tor", where you can have lunch, icecream, or a drink, stroll down the pedestrian zone of Brandenburger Strasse towards the Church of St. Peter and St. Paul, turn left and take a look at the Nauen Tor. . .
. . . before you explore "Little Amsterdam" or the so-called "Holländische Viertel", built by Frederick II. to accommodate Dutch craftsmen and their families.
Today, the reconstructed houses contain beautiful stores offering arts and crafts,
or house restaurants offering international as well as local specialties.
At the end of your stay in Potsdam, you can take the "Strassenbahn" to Potsdam Main Train Station, from where a 15 - 20 minute S-Bahn ride gets you back into the heart of Berlin.
All images by courtesy of H.F. and J.B.