The capital of Poland, Warsaw is often overlooked as a destination - a lot of Poles will even say you should give it a miss and head for other cities like Kraków instead. If you like exploring forgotten corners of cities, though, Warsaw has plenty to keep you occupied. The city and its people suffered badly during World War II, then came under communist rule between then and 1989, both have which have left very visible traces. Warsaw is now changing rapidly, with new building projects appearing across the city as the result of renewed interest in the city from both national and global businesses.
If you like rail travel, the Berlin-Warszawa Express will take you there from Berlin Hauptbahnhof in around 5 1/2 hours for a fairly reasonable price (60 Euros at the time of writing, if booked far enough in advance - you can check bahn.de for the latest prices).