A note on public transportation in BCN – if you plan to use the metro, which I highly advise because trains go almost everywhere and run in very precise intervals of about 3 minutes (Dear DC metro, why can’t you be this efficient), get a T-10 fare card. The T-10 cost 9.45 euros when I visited in November ’12 and allows you 10 trips (including transfers) on the metro, buses and trams to any destination, including the train that runs from the airport to city center. I believe there is a window of time to transfer from the metro to a bus or tram and vice versa. A one-way trip on the metro is 2 euros. You do the math.
The BCN public transportation system, or Transports Metropolitans de Barcelona, also has unlimited daily travel passes, but the T-1o is really the best buy if you’re going to be there a week-ish like me. It even tells you on the back on the card how many trips you have taken. One thing I feel I should note, especially because I depend on my credit card for everything – many travel sites say the metro station fare vending machines take “card” and cash. This is partly accurate. They actually do not take credit cards, only debit/bank cards/cards requiring a PIN. I was really stressed by this because I had forgotten my debit card, but thankfully I had enough euros to purchase a T-10 fare card!