A day-ticket (starting from 0:00 am to 3.00 am of the following day) for the farezone AB (this is the city-region of Berlin) is 6,30 Euro.
You'll find a tourist-ticket for 48 hours (Berlin CityTourCard AB 16,90 or a tourist-ticket for 72 hours Berlin CityTourCard AB 22,90) as well. These are valid for 48h or 72 h starting from that moment where they are clocked in. You'll find more information on public transportation on the english website: http://www.bvg.de/index.php/en/index.html