Must see sights in Barcelona include :
Las Ramblas - just walk along & look at the statues ( people pretending to be statues )
Open top bus tour is brilliant if you get time
Have a coffee ( cafe con leche ) in restaurant on top floor of El Corte Ingles in Placa De Catalunya for fantastic views of the city . This is a great freebie ( although coffee isn't free - approx 1 euro 30 )
Nou Camp football stadium tour if you like footie. This can be done via open top bus by the way as it stops there
Olympic stadium tour. Home of Espanyol football club, although it's not all about football & also a stop on open top bus.