Great city, lots of history
I loved Berlin, not least for the fact that history fascinates me. Planned on staying 3 nights, ended up there 6. Definitely go to the Holocaust Museum (located underneath the Memorial to the Murdered Jews of Europe) and the Topographie des Terrors Museum - both are free. Also do a pub crawl or two.
Insider Tip:Make sure you go to Konnopke's for some currywurst - best in town!