We bought a travel card at the nearest metro station, less than £10 for a week and covers the metro, buses and trams but not the hop on/off tourist bus. If you are buying one of those, take your passport with you when you buy it so they can add your ID to the ticket, there are no swipe machines like the London underground so if you do get stopped and checked your ID is on there. We used the metro every day and used the tram to go along the riverside