Berlin Busses
Buses are of course the slowest public transport vehicles, but they will get you in every corner of Berlin.
Besides the normal and metro busses there are also express busses (marked with a X), which don't stop at every station.
The most famous bus line especially for tourists is 100, which leaves from Zoo Station, or
if you want to go the other way round - Alexanderplatz,
and crosses most of historic Berlin, including many of the sights.
For the price of a city bus ticket or daily pass it's possible to see much of the city from one of these double-decker tour buses. Sit up top as it's easier to see the Reichstag, as well as the many historic buildings on Unter den Linden.
If you're lucky, you'll get the legendary bus-driver who delivers a commentary (in Berlin-accented German) on the trip.
Line 200 takes nearly the same route, but it goes through the modern quarters around Potsdamer Platz.
Either ride is a must do on any trip to Berlin.
Some bus companies do offer the City-Circle-Tour, where you can hop on and off the bus at 12 different stops.
Daily City Circle sightseeing tours with doble-decker buses (open roof in summer) in 8 languages starting every 15-30 minutes.