The bus tours are good to do they show you round the city - and they even go to Camp Nou (do the stadium & museum) which is fantastic and you can get on and off when you want, they run until late evening.
I did one of those bus tours on a day trip from Salou. The Nou Camp was quite impressive although it was being prepared for a 3 tenners concert.
The Gaudi cathedral is quite a sight as well, especially from up in the mountain/hill side, there is some kind of rock ledge that they call "The balcony" or something from which you can look over the whole city.
They also took us to the olympic village, and whilst there I manged to slip badly twisting my ankle causing it to swell up to the size of a large orange. I was pretty much out of action for the rest of my holiday which didn't stop me from drinking heavilly whilst sat by the pool with me dodgy ankle up.
It was OK all in all, Barcelona has some well impressive architecture, in particualy The Gaudi and colinial buildings.
Just a shame the Spaniards where a bunch of cunt's. I found the whole thing very similar to Paris myself only with beaches and hotter.