Barcelona is AWESOME no matter what you're there for!!!
If you've never heard of Gaudi, his architecture is all over Barcelona and it is amazing. This page has links to some photos.
Of course, the photos don't nearly do them justice. I never visited Park Guell, but I wish I had. There is also a Salvador Dali Museum in Figueres that you could do on a day trip. It's worth the effort. Keep in mind that most museums are closed on Mondays in Europe.
Food is available on La Rambla which is a busy street where there are plenty of people, street acts, and stuff to buy. You can look it up on Google. There are tons of tourists also so beware of pick pockets. They tend to hang out around signs that say "beware of pick pockets" because people immediately touch the pocket with thier wallet. DO NOT PUT YOUR STUFF ON THE GROUND!!!
If you're looking for nightlife ask the locals. They'll direct you to the non-touristy spots where they party until 6-7AM. It's pretty safe, but I wouldn't go out alone late at night. The people at the clubs are generally very friendly (if you're Canadian at least).
The beaches are nice, but I'd consider visiting Valencia for a few days if you're planning to soak up the sun. There's a great hostel right on the beach there for about $10 a night.
And save your $ on the first class rail. This is backpacking dammit! If you want to go first class stay at the Hilton.