In fact, why not rent a bike and take a day trip by bike to Potsdam. There are plenty of parks and lakes to criss-cross. Plus everybody cycles in good weather. However, you don't have to cycle all the way. Take the bike on the S7 Train, change at Wannsee and get the S1 to Potsdam (just cross, it's on the same platform).
Potsdam is a World Heritage site with beautifully restored period architecture. It lies just 15km (10 miles) south-west of Berlin, but it is almost like travelling to another Eastern Europe city altogether, on par with Prague or Budapest, just a tad smaller (this is no exaggeration). Potsdam has a picturesque old town with many (outdoor) cafés, and even has its own Brandenburg Gate.
It is also home of the Royal Gardens, and the popular Sans Souci Palace. There is proof of Russian culture to be found in Potsdam's "Alexandrowka" which has replicas of Russian village houses and a pretty Orthodox Church on the Hill, and Dutch period buildings fill the Dutch Quarter (Holländisches Viertel). The historic sections of the city provide the flair of a city steeped in European tradition.
Bike rental information: www.berlinfo.com/Traveltime/WithinBerlin/by_bike/bike_rental/index.htm
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