Hi,
Aside from the historical suggestions given above, lots of historical "stuff" can be seen in Greater Berlin on WW II and German history, depending on your level of interest. But "huge history buff" tells a lot. With the days you have in Berlin you can devote all of it tracking down sites pertaining to WW 2, especially if one of the days is a Sunday.
On Sunday the German History Museum (DHM) shows documentary films on an aspect of the war. One Sunday as you walk up on Unter den Linden to Museum Island, turn left, you'll run into something like a "farmers market" Go all the way, since towards the very end is a large display table upon table , where used books are sold. The books are almost exclusively on war history or WW2, practically 98% in German, originals, the remainder are in English, the original language of the work, or English translations from German or French....just depends what your luck is.
I would suggest to a "huge history buff" going out to Berlin-Karlshorst (S-3) to the war museum, ca 20-25 walk from the S-Bahn station Karlshorst, there is a bus also going right there. It's been redone, modernized, saw it this June, the last time there was in 1999, the focus now is exclusively on the Eastern Front, from the Nazi-Soviet Pact to the fall of Berlin...well worth your time but I thought it could have been more critical in the historical approach.
Take a day trip to Frankfurt an der Oder, get to the Oder River bridges , (die Oder-Bruch) , ca 40 mins walk., and just imagine in 1945. Another site is Seelow, where the battle for Berlin began, the Russians launching a frontal attack. You can get to Seelow from Frankfurt an der Oder by S-Bahn. The Battlefield Memorial Museum is there, mostly in Russian and German, some in English. (Gedenkstätte und Museum Seelower Höhen).