It's unnarrated, but Berlin's cheapest bus tour is on the double-decker N°100 and 200 buses. These run between Alexanderplatz and Zoologischer Bahnhof, with N°100 passing through Tiergarten and past the Reichstag, and N°200 taking a more southern route via the embassy district and Potsdamer Platz, before both lines converge on Unter den Linden.