Parc de la Ciutadella

The Arc de Triumf outside of the park. They built it for the world fair that Spain hosted a long time ago.
The Museum of Zoology in the park.
The really pretty Gaudi Fountain/Statue that everyone goes here to see. Definitely worth going to. There are also tons of really pretty trees and flowers and fountains all throughout the park.
The pretty lake in the middle of the park that you can rent rowboats for