The Berlin WelcomeCard is a combined transport and reduction card (museums, bike tours/rental, boat tours, etc) valid for zone AB or zone ABC (which includes Potsdam and Schönefeld airport). Cards are valid for 48 hours (AB €23 / ABC €28), 72 hours (€33/38), and 4 (€40/45), 5 (€46/49) or 6 days (€49/52). There's also a 72-hour variety (€51/52) that includes free admission to 5 museums on the  Berlin museum island in Berlin Mitte. Cards are sold online and from BVG ticket machines, tourist offices, S-Bahn offices, hotels and kiosks. The similar Berlin CityTourCard (www.citytourcard.com) is a little cheaper, with restaurant, bar and club discounts geared towards younger travellers; a 48-hour ticket costs €19,90 for AB, €22,90 for ABC, and there are also 72 hours, 4, 5, and 6-day tickets.