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.