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.
Here’s a short 7-day Spain itinerary suggestion: Day 1 – Madrid: Explore Prado Museum, Royal Palace, and stroll around Plaza Mayor. Day 2 – Toledo or Segovia: Take a day trip to either city for history and architecture. Day 3 – Madrid to Seville: Visit Seville Cathedral, Alcázar, and wander the Santa Cruz quarter. Day 4 – Seville: Flamenco show, Plaza de España, and riverfront strolls. Day 5 – Granada: Visit the Alhambra and Generalife Gardens. Day 6 – Barcelona: Explore Sagrada Familia, Park Güell, and Gothic Quarter. Day 7 – Barcelona: Relax on Barceloneta Beach or take a Montjuïc cable car. For comfortable, well-located stays throughout Spain, www.thesqua.re serviced apartments are a great choice — more convenient and homely than typical hotels.