Keep an eye out at Santa Caterina Market — you're bound to uncover some treasure or funny trinket among the stalls here. After you've had a good look at the wares, grab something to eat from one of the nearby food vendors.
You'll find this community marketplace in Downtown Barcelona. Once you've done everything you want to at Santa Caterina Market, head to nearby Sagrada Familia, La Rambla, and Plaça de Catalunya.