We're going to be in Berlin for about 8 hours and would like advice about how best to see the sights.
Exactly which 8 hours are these (i.e., what time of day)? Is this part of a Baltic Cruise, so you're coming from Rostock or Warnemunde, or is it something else? When is the trip?
These details will help people give better answers.
Take a look at one of the walking tours like Original Berlin Walks. They have a 4 hr walking tour that takes you to most of the major tourist sites(outdoors only).
I can't see doing a Hop on Hop off not unless it is bad weather. Last April I walked from the Alexanderplatz (that is where the big tv antenna is located ) to the Reichstag Building. That route covers a lot of sites including the Berliner Dom the Museum quarter and the Brandenburg Gate. It did not take that long. It is flat. You can do that yourself using a guide book instead of a group