Take the bus number 100 from Alexanderplatz to Zoologischer Garten (or the other way round). From the upper deck, you have excellent sightseeing options. Hop on and off whereever it stops. You will see Berlin's major sights.

The fare is 2.30 Euros per person (or free, if you buy a ticket for several days).

www.bvg.de

Another option is a boat tour on the river Spree. Board the boat near the Museum Island.

If you want to do a tour of the major sites in Berlin I would recommend a walking tour. We did a 4 hour walking tour. It was excellent!! After the tour we took the #100 bus to revisit some of the sites we wanted to spend more time at.

Thanks for the responses. Snowflake 25, what was the name of the tour you went on and where did it go from?

I was going to suggest a walking tour also, it gives you a great orientation of the city. There are multiple companies in Berlin, all offering similar tours and all with pretty good reviews, so you get a quality tour with them all. We went with Insider Tours for our Berlin walk, plus their Sachsenhausen tour and Potsdam tour, but there is also Original Walks, or Fat Tire bikes, or even Segway tours which might be fun, though pricey. Trip Advisor lists them all along with their reviews.

www.insidertour.com

Another +1 for the walking tour - it's absolutely terrific. See www.newberlintours.com for details. For thie one, there's no need to book and the tour is free - you just tip the guide at the end.

Well, then it isn't "free" is it? You don't need to book with the other tour companies either, you just need to show up at their meeting place, and the tour groups are going to be a lot smaller than the "free" tours and present a higher quality tour.

Check the reviews on TA and see where the free tours are listed - #31 compared to the other tour companies which share the top 5 places. That should tell you something about the quality of the tours. They just had a big scandal about them on German TV a month or two ago, Rick Steves had a big blog section about the ethics of the company policies, the treatment of the guides, etc.

We opted for a private tour guide and it was a highlight of our stay. We generally treat ourselves to a private guide when visiting a complex and interesting city such as Berlin as we find it a great investment. Our day was tailored to our specific interests and our guide, Jeremy Misberg, was excellent.

We are headed back to Berlin in November and have booked with Jeremy for another day to see the many places still on our list.

We did the Insider tour. Our tour guide was excellent, very knowledgeable and spoke with great passion about the history of the city.