Sites in Berlin

Brandenburg Gate, Memorial to the Murdered Jews of Europe, Reichstag, Topography of Terror. Don’t waste your time visiting Checkpoint Charlie.

Ok, Thanks!

Good list from Suki if WWII and Cold War history are your main interest there. Berlin also has some world-class museums. with ancient art and architecture from the MIddle East and elsewhere. Here are our host's suggestions ("At a Glance"). Look them over, decide what matters most to you, locate those places on a map, compare it with the HOHO bus route, then decide whether to use the HOHO or local transit to get where you're going. A lot of the main sights are within a mile or so of each other so you can do much by foot. Come back for more.

I enjoyed the HO HO bus tour. I agree about skipping Checkpoint Charlie.

Do a good walking tour instead of the ho-ho bus. You just spent a lot of money to get here, and a good tour will make it worth while. If you aren't sure, read the reviews for the ho-ho buses on Trip Advisor and then read reviews for the most popular walking tours. See what the difference is.
Personal favorite is Insider Tours, but there are others that are good too. Most cost around 20€.

Recommendations are always better if we know your interests. I'm guessing Berlin is not a place in which you are super interested if you are just spending a day there. Count me among those that think a walking tour would be a good use of your time. If you have specific interests, let us know and we might be able to help more. For example, if you are really interested in the Berlin Wall -- go to the Berlin Wall Memorial (Gedenkstätte Berliner Mauer ). Are you really interested in old East Germany life? Go to the DDR Museum near Museum Island.

Hopefully this is one full day ,say at least 12 hours in the summer.

if your main interest is history, I suggest that you focus on only exploring east of the Brandenburg Gate and the eastern districts, even though there are sites related to Prussian history in western Berlin as well, as are WW2 sites.

Reichstag tour is superb.

Reichstag tour is a good idea, too. i would suggest doing something that lets you see Berlin from above in the evening. Think about the Reichstag tour OR dinner in the TV tower restaurant (pay extra for a window seat) right before sunset. Hang around long enough to see Berlin from above during daylight, during twilight, and after dark.

maddyaduke, have you visited anywhere else in just one day? What did you do?

Berlin has a very diverse variety of what to see: pre-WWI, during WWII & Holocost, afteward with East-West, then Reunification and the modern, vibrant city Berlin has become. Variety is also true for museums and the genres of their respective collections. Ditto for music, food, etc.

For me, my one day planning always begins with impressionist art (i.e. Barbarini Musuem in Potsdam) and anything that I consider to be truly unique to that city. For example, Berlin had the wall and that is exclusive to Berlin. What is special to you?

Do you need to pay an admission/ticket to walk through ZooGarten? I am planning on walking from Kaiser-Wilhelm-Gedächniskirche to Siegessäule "Victory Column".