If there is one sight you shouldn`t miss for, it must be the "East Side Gallery" - what the remaining 1000-or-so metres of the Wall are called nowadays. As you see this symbol of former authoritarian dictatorship and totalitarism, it really makes you value your freedom (cliche!).
My mate Keith got a DDR stamp in his passport for (for DM1!) in the souvenir shop around the back. Novelty gift!
Location: Take U1 to Warschauerstrasse. You can also start from Ostbahnhof (East Railway Station) because it`s quite near.