I would be sure to go to Barcelona. You would need at least 2 full days there to see the sites. Very interesting architecture. If you are into art Madrid, Madrid has some famous art museums. For beaches and sun go south.
I think all cities in this wonderful country are beautiful, and if you visit any of them, you will have a lot of fun. I have only good memories of Spain, except for the museums and architecture. For example, recently in Spain a bookmaker opened https://betmaster.lat/, and I won a very good amount of money.