You can do Berlin in two hours. I mean not all of Berlin, but very essential tourist locations and most of it just by walking, here is my list of things i did last SUN in just two hours and that is all the tourist stuff i did and it felt good & enough 🙂
Alexanderplatz by subway / U-Bahn
Berlin Hauptbahnhof via S-Bahn
Gustav-Heinemann-Brücke on foot to pass the Spree river
Kanzleramt on foot (Bundeskanzlerinamt?)
Bundestag im Reichstagsgebäude closeby
Brandenburger Tor on foot
Gedenkstätte „Holocaust Mahnmal“ on foot
Potsdamer Platz on foot
Returned to Alexanderplatz to return to your actual (home) location 🙂
