The best way to do sightseeing is on top of a double decker bus. Try to get seats in the first row for a great panoramic view. This has an added benefit: more legroom.
For the price of a single fare busticket, you can see a lot of Berlin's sights without having to go on a guided tour. Take Bus 100 (Zoo Station), go on the upper deck, enjoy. The bus takes you into the eastern part of the city as well.