M41 metrobus runs every 10 minutes and basically gives you free city tour. Starts hauptbahnhof, passes reichstag, brandenburg gate, then through kreuzberg to sonnenallee

Sit right side for best views of government buildings. Takes 25 minutes end to end. Way cheaper than hop-on-hop-off buses, you're riding with actual locals

Regular transit ticket works - €3.40 or use day pass. Skip expensive tourist buses