Berlin is a city which I would absolutely love to re-visit. It's a place full of history, culture and action. If you plan on visiting, I highly suggest you get a Berlin Card, which allows you to use the public transportation system (S & U Bahns) for an unlimited time, depending on which type of card you purchase. The card also gives you discounts at certain musuems, such as the Deutsches Historisches Museum, which was fascinating. If you're interested in learning about the history of Germany, you should consider it. There was quite a lot of information about the rise of Hitler, World War II and the Holocaust, of course.
The Reichstag, Brandenburg Gate, and the Jewish Memorial were also great places to visit... and the list goes on. Berlin is a city that's great for walking. Since it is the Christmas season, the city was filled with Christmas markets. I am a great fan of markets, and needless to say, I was having the time of my life! The Germans certainly know how to celebrate the holidays. The atmosphere was so wonderful at the markets. Everyone was sipping glühwein (a red wine), eating bratwurst (they are so good!), and enjoying the company of their family or friends. Germany is definitely a great place to visit during the Christmas season.