Great city - I need to go back!
I was able to train to Berlin for free during the Sochi Olympics, so I only had one day to see as much as I could. I hit all the big stuff - Brandenburg Gate, Reichstag, Checkpoint Charlie, the Topography of Terror museum, the Soviet Memorial, and the Memorial to the Murdered Jews of Europe. The transport system is excellent, and even in February the weather was mild enough that I could get an early start on the city.
Insider Tip:Buy the Day Ticket (Tageskarte) for the metro, for zones AB for 6.70 Euro. You can go literally anywhere in the city all day, just pick a train and hop on. Don't miss the Topography of Terror museum - it's emotionally moving on a number of levels. DO miss Checkpoint Charlie right down the road - it's just an overglorified souvenir checkpoint with a couple of reproduced signs.