If you put "Berlin" into the search and select "thread titles only" you'll find several threads on the city.
The whole city is a great location.. old, modern, historic, contempory, people, architecture.
The Reichstag and the government district are interesting, there's been some very modern development along the river in that area and along towards the new Hauptbahnhof. For communist era architecture a walk along Karl Marx Allee with the pepperpot developments. Modern plattenbau at Marzahn, but it's probably not an area to be in your own if you're new to the city.
The Fernsehturm is an iconic landmark, there's a great evening view of it from the bridge above Warschauer Strasse S-bahn station (and that's a convenient area for some really good cheap eats if you head towards Simon Dach Strasse or try Hirsch on Kopernikusstrasse west of Warschauer Strasse). There's a closer (and higher) view from the observation platform of the Park Inn hotel at Alexanderplatz.
If you want an idea of what's available in Berlin, watch Run Lola Run and The Bourne Supremacy.