Barcelona has lots to see and do on it's own. The Gaudi house, La Sagrada Familla, Gaudi Village, Picasso museum, Salvadore Dalli museum, Barri Gotic, and great Tappas and other excellent restaurants. The Roman underground is also worth checking, and take the train North to Figueres and also go South to Montserrat. Use Rick Steves book and also go to virtual tourist. Go to this site to check out more on these things.
Barcelona Travel Guide, Catalonia, Spain
I spent a week in Barcelona and it was not quite enough time to see everything there.