It's a cliche, but you have to take the #100 bus from Zoologischer Garten to Alexander Platz. It's a cheap tour of the major sites of the city. Even better is to take the nightbus equivalent of the #100 (I don't know which that is now). It's cool to ride down Strasse des 17. Juni, past the Reichstag, under the Brandenburg Gate and down Unter den Linden in the middle of the night at the top of a double-decker bus. It's very eerie, but cool.

Basically, taxis are kind of expensive in Berlin, but if you´re only going a short distance (i think it´s 5 minutes or 2 km), then say you want the 5 Mark Tarif. This means that it´s a fixed price of 5 marks. This is probably around 3-4 marks cheaper than paying the normal fare... Sometimes the taxi drivers get annoyed at that, but well, too bad for them...