Hi,
I am from Barcelona, and i have found almost all caches there (my user id here in the forums is different from the one of the logs),
so I will give you my opinion:
Parc Güell is very recommendable, and Rambles de Barcelona is a site where you must go if you go to Barcelona, so get the cache in that moment.
Cul-de-sac by garri (GC1552B) is quite new and very original, but a bit (not very) hard.
Barcelona Invasion by mcruley (GCJ1NG) is very good but it take lots of time (it took one morning and two afternoons to my team) .
But depending on the days you are staying there, and your preferences, there is a very singular mountain at about 40-50 km from Barcelona, Montserrat ( http://en.wikipedia.org/wiki/Montserrat_%28mountain%29 ), and do some cache there, for example, St. Miquel - Montserrat by Jucar (GCMEF2), there are some more, with different difficulty.
If you go by plane, there is also a TB hotel in the airport at about 30 minuts going and returning.
In "Barcelona" virtual cache you will get nice views of the city.
Greetings from Barcelona and enjoy it!