I have tried to visit the best sights in Berlin during my week in the capital of Germany which are reachable by public transport – metro or bus – and there is plenty to see. You need at least a week, if you want to stroll through all the museums and historical sights in Berlin. I visited the Weltzeituhr at the Alexanderplatz, Deutscher Dom, Concert house, Französischer Dom, Berliner Dom, Grave of Friedrich I, Altes Museum, Checkpoint Charlie, wall and border stone of DDR, Jewish museum, Berlin Tempelhof, remainings of an old amusement park in East Berlin, Spree, alte Nationalgalerie, the Ishtar gate from Babylon in the Pergamon Museum, Strasse des 17. Juni with the Siegessäule and the Brandenburger Tor, Russian War Memorial in the Treptow park and the Kaufhaus des Westens.
Berlin was really worth to visit for sightseeing. Plenty of restaurants, museums, and entertainment keeps you busy well over a week. Berlin is also a quite international city even though you can find it hard to find signs in English when using public transportation.