Catch the S-Bahn to Wansee and hire a bike from outside the station. They supply you with a map to make your way around the lake to Potsdam.
It is a beautiful ride through woodland and along beautiful lakeside scenery. Stop off at the House of the Wansee Conference on the way. This is where the Nazis decided on the "Final Solution" for the "Jewish Problem." It is truly horrific how such a beautiful building in a beautiful location could be the setting for such an abhorrent moment in history.
On the way to Potsdam, there are plenty of peaceful places to stop, including a couple of lovely cafés with large lawns. Potsdam is beautiful, and the main park there is the big attraction with its many palaces and other architectural wonders and landscaped gardens.
Cycle back to Wansee, or, if like me (lazy), put the bike on the S-Bahn in one of the special cycle carriages!
From journal 3 Nights in Berlin