Suggestions for Berlin walking tour

Take the U8 to Bernauer Strasse. Walk to the west through the park paralleling Bernauer Strasse. This is along the site of the Berlin Wall with many interpretive photos and displays on the history of the Wall (most in English). When you get near the old Nordbahnhof, there is a section of intact wall. Directly across Bernauer Strasse is the Berlin Wall Memorial and Documentation Center. From Nordbahnhof, you can use the S-Bahn to move on to your next sight in Berlin.

Take a U or S line to its terminus and wander around to see what you can find.

The Berlin city walk in the Rick Steves Berlin and Germany guides are good.

Insider Tour gets good marks for group tours. https://insidertour.com/tours/

If a private guide is in your budget (and if he is available), I would recommend a half to whole day with Robert Sommer. He was the 15-year-old punk-rocker son of a high-ranking East German bureaucrat the day the Berlin Wall fell. He can tell you firsthand about growing up in East Germany, his experience of the fall of the Berlin Wall, life as a squatter in the former East Berlin, clashes between punk rockers and neoNazis, re-unification, and modern Berlin. He's a very interesting guy who ended up getting a PhD in history and is married to a former member of Germany's Parliament. I'm guessing I've done well over a dozen tours with him -- everything from a standard Intro to Berlin tour to a post-WWII East vs West architecture tour to a tour of Cold War era underground bunkers that held Soviet nuclear warheads. http://thetrueberliner.com/

History... so many options... a sampling...

Head to Treptower Park to see the enormous Soviet Memorial built on/around a mass grave for around 6,000 Russian troops.

To me, one of America's greatest moments is the Berlin Airlift. Visit the decommissioned Tempelhof Airport and consider a tour. Visit the nearby Berlin Airlift Memorial. Leave a flower for the airmen who lost their lives during this operation. If you are not familiar with Gail Halvorsen, the "Candy Bomber" who started dropping candy to the children of Berlin during the Airlift, google him and read a bit about him.

If you are a fan of Dietrich Bonhoeffer, take a train out to Grunewald Station on a Saturday morning. Pay your respects at Gleiß/Track 17 to the Jews deported from Berlin during WWII -- there is a grate on the platform for every train that left the track carrying Jewish people. Then walk through a great early 20th century neighborhood to the home of Bonhoeffer's parents and his home in Berlin during the war. It is where he was arrested by the Gestapo. There is a 1-hour English Tour on Saturday mornings, but it is possible to schedule a tour at other times if you are not in Berlin on a Saturday morning. https://www.bonhoeffer-haus-berlin.de/en/

The website of Berlin offers various self-guided tour suggestions:
https://www.berlin.de/en/tourism/self-guided-tours/

Maybe already mentioned: Berlin covers a huge area, so better combine walking with other tour forms and public transport.

I really like Insider Tours for a group tour, with various themes. Have been on 5 of them so far. For private, I can recommend Jeremy the Berlin Expert.
You will get so much more out of your time here if you go on some sort of walking tour with a guide that can answer questions.