It has been mentioned briefly in an earlier post, but I think visiting some (or all) of Antonio Gaudi's work in Barcelona is definitely worth while. Be sure you don't miss La Sagrada Familia and Park Güell.
Even if you don't like architecture, these sights are so different than anything I've seen making it fun and interesting visiting them.
There are many routes that you can walk that will bring you to many of Gaudi's creations.

For food, drinks and just looking at people I would recommend the Ramblas in Barcelona.