I spent a week in Berlin 7-15 May 2025. It was my first time there. Berlin is a big, impressive city. A week isn't nearly enough time to see it all. I did Rick's Berlin walk from the Reichstag to Alexanderplatz and visited numerous places on and off the walking route. Unfortunately, the two museums I most wanted to see, the Pergamon and the German History Museum, were both closed, for a year or two, for extensive remodeling. The art museum (Gemäldegalerie) and nearby music museum (don't miss if you're a musician) were excellent. Berlin's main train station, the Hauptbahnhof, is huge and impressive, but quite easy to navigate because it's so open and airy. You can see where you need to go from almost anywhere in it. Besides trains, it has many stores and eateries. I was most touched at heart by the Berlin Wall Memorial (see Rick's Guidebook) and Tempelhof Airport, site of the Berlin Airlift. Everything I have mentioned is easily accessible using Berlin's excellent public transportation system. By all means, get one of the multi-day passes.
I only had one major problem in Berlin - getting downtown from the airport. The S9 train, recommended in the Guidebook and by my hotel, was shut down for rail replacement and there was no notice of that, at least not in English, at the airport. I got on it but it stopped almost immediately and we had to get off and walk to a bus workaround. The bus meandered through little towns, got stuck in traffic jams, and had no information as to where to get off. In the end, it took me over 4 hours and several different trains in addition to the bus to get from the airport to my hotel in Prenzlauerberg. Lesson learned: ALWAYS consult Google Maps or similar before starting a trip on public transportation. That goes for other cities like London or Paris too. The app knows what's running and what's not, where the delays are, etc. Doing that, I had no further problems. My trip back to the airport, not on S9, took less than an hour.