What Should I See in Barcelona?

Borrowed from http://www.barcelona-turist.se/sagrada.htm

My vacation has now left the familiar path and will tomorrow be taking me to Barcelona. I have never been to Spain/Catalonia and am more than excited! I have about 5 days there, so what do I need to see?

I have heard of Gaudi’s architecture and the Museu Picasso.

What else should I see, eat, drink and not miss?