To get to the city center of Berlin you can use the subway U6 which takes you directly to Friedrichstraße and Unter den Linden, a main boulevard with many museums and historical buildings, stretching from Brandenburg gate to Berlin Cathedral. If you hop off earlier, you will be in the hip district of Kreuzberg with lots of restaurants, cafes and bars or you can fly a kite at Tempelhofer Feld, which used to be a city airport. Besides the many museums, sightseeing attractions and culinary spots, Berlin offers lots of green and swimming lakes in and around the city. By train you can do day trips to the surrounding areas and for example visit Potsdam with its castle "Sanssouci", "Tropical Island Resort"or one of the many lakes in Brandenburg.