One of the most amazing cities I've ever seen. Check out as much Gaudi as you can, then cathedrals, museums (Picasso is a must - even if you hate modern art you will appreciate the work he put into it). there is literally no end to the things you can do and see here. Hop on Hop off tours are great, as the city is so big.
Beautiful city. Plenty to see just walking around or go out further to Monte Carlo or the surrounding area.