The Traveller Recommends buying a T10. Most locals using public transport in Barcelona pay with a T10 card (Zone 1), which allows for 10 journeys, costs €9.95 (2016) and is multipersonal: so for example a couple can use the same card for 5 journeys together, passing it through the ticket machine twice on each entry to the subway, bus or tram. A particularly good thing about the T10 is that you can change transport modes (subway, bus or tram) during the first 75 minutes of each journey without the card deducting another of its 10 journeys, although you must always pass the ticket through the machine (in the direction of the arrow on the card) every time you enter a bus or tram: if not, and you are caught, the fine is €100. It is common to have your ticket checked by roaming bands of inspectors accompanied by security guards. Note there is the slightly complicating factor that you can’t enter subway stations twice, or use the same numbered bus or tram twice, during the same 75 minute journey without a second journey being deducted when you pass your T10 through the machine. Using the T10 is easy and you will soon be like everyone else in effortlessly moving through the transport system: once you are used to it the T10 really is your passport to the city. Buy your T10 from subway stations (at the window or in ATM machines that function in Catalan, Spanish and English), or at machines in the tram stops. You cannot buy them from tram or bus drivers.
Other options include the day pass card (T-Dia) and the 2 to 5 day passes (HolaBCN: see Arrival). If you are less mobile and prefer the ease of being a hop-on hop-off visitor then another popular choice for seeing the tourist sights is the open top double-decker Bus Turístic which costs €28 or €35 for 1 or 2 days and follows three routes (blue, red, and green). The bus can get full (the ticket does not guarantee you a seat), the queues long at peak times, and busy city traffic can at times make the wait long, but many people seem happy with this choice. The routes comprehensively cover all the main sights and The Traveller Recommends a 2 day ticket rather than trying to pack everything into one day. Another typically tourist option is the Barcelona Card which offers free city transport plus various discounts, and in 2016 costs €45-60 for 3-5 days. Finally, yellow and black taxis are plentiful in the city and, in The Traveller’s experience as a local at least, do not rip you off.