La Sagrada Familia is one of Gaudí's most famous works in Barcelona. It's a giant Basilica that has been under construction since 1882 (that's not a typing error!) and it's not expected to be completed for some time yet.
Take a walk in Parc Güell, which is definitely more than a park. It is a symbol of Barcelona. Unique in the world, Parc Güell is one of Antoni Gaudí's most emblematic works and it's absolutely worth the visit !