Almost all popular sights of Berlin are located in the quarter "Berlin-Mitte" in the center of the city. If you want to do sightseeing in the summertime i recommend to rent a bike and just cycle around - like i did in this Video ;)
The itinerary shown in between can be found on the website mentioned below.
BTW: I rented a bicycle by using the "Call a Bike" system of the Deutsche Bahn (just Google for that for more informtion)
Have fun cycling through Berlin!
"Wednesday Morning" by slackstring