Visiting tourist attractions
In addition to the Sagrada Familia, the Park Güell (which I like much less since you need to buy tickets), Casa Batlló, Casa Milà more often called La Pedrera there is also since 2017 a new attraction Casa Vicens. If you would like to see two, I would go to see Sagrada Familia because of the fantastic light and the apartment in Casa Batlló.
There are many art museums in Barcelona but in addition to the best technical museum Cosmo Caixa there is also a nice Maritime Museum mmb or if you would like to see something outside Barcelona go to Terrassa which has the MNAC.
I also like the Moco Museum of modern art with many interesting artists (i.e. Guillermo Lorca)
The Picasso museum has beautiful architecture and Fundació Joan Miró is another interesting place to go.
Everybody who come to visit Barcelona knows about Gaudi and the Sagrada Familia but some people decide not to visit the inside because of the cost. I think it is still worth it to go because of the incredible light inside the cathedral. You can buy tickets on the internet (https://sagradafamilia.org/en/).
