If you're not into walking and prefer to save the money for hop on hop off, this one's for you. Go to Alexanderplatz, get a all day ticket for 7 euro, catch the bus number 100 and let it take you through some of the most important Berlin sights, like Alexanderplatz, Unter den Linden, Brandenburg Gate, the Victory Column and Zoo in Charlottenburg. Basically you will visit Berlin from the center of the East to the center of the West.
It's a double decker, so if you're lucky enough to snatch the front seat up top, you'll definitely be in for a ride with a view!
Check also
What to do in Berlin: Tips for all kinds of visitors
What is it and how worth it is the Berlin Welcome Card?