Damn Willy, that's a pretty sweet deal.
I've been to Madrid, Barcelona, and Costa Rica, and I had a blast in all of them. However, I would steer clear of Barcelona for the following reasons:
1) You will have much less opportunity to practice your Spanish outside of the classroom in Barcelona than either of the other two places I mentioned.
2) You could have a great time in all of the above mentioned places.
Costa Rica is quite inexpensive (at least it was when I visited), has beaches, rain forest, dry forest, and volcanos - all within short trips of eachother. Also, I found the natives to be very friendly and quite patient with my Spanglish.
Madrid is a magical city, that, to me, really felt more like old world Spain, rich with history, culture, and Espanolitas! I also had a good experience practicing Spanish there.
Barcelona is full of people from all over the globe, and a great place to get drunk. I found that I was usually cut off about halfway into a Spanish sentence & spoken to in English, which didn't do a whole lot of good for me educationally. If you do go to somewhere else in Spain you MUST visit Barcelona to see Gaudi's architecture, however, it is breathtaking.