Kreuzberg, Potsdamer, Parisier - there's endless things to see and do!
Berlin is a fascinating place - you've got the hip Kreuzberg neighborhood, the amazing museum island, walks through Potsdamer Platz down into Mitte and onward to Alexander Platz...if you do it right, you can take a day for each section of Berlin and easily spend a solid week seeing new things. There are a ton of museums in case it's a rainy day, but if it's nice enough you can stroll for hours and see most of the major sites, especially the Berlin Wall.
Insider Tip:Check out the Museum of Musical Instruments, housed next to the Berlin Symphony building, a few minutes' walk from Potsdamer Platz metro station! It's a fun thing to do if the weather's not excellent, and they have audio clips of instruments from the 16th century and earlier. Also, check out the "Museum of Things" in Kreuzberg neighborhood - a really eccentric collection of designed objects from the 19th century until today. Both great places to check out in between the more typical tourist sites!