In Andalusia (Southern Spain): Seville (Cathedral, minaret Giralda, Alcázar par of Game of Thrones was filmed there), Cordova (Roman bridge, mosque-cathedral), and Granada (Alhambra, Tombs of Isabella and Ferdinand). You must book the Alhambra now, only 5,000 tourists allowed every day.
Roman bridge Cordova
Mosque-cathedral Cordova (Córdoba in Spanish)
and if you have time near Cordova you should take a bus to see Medina Azahara
https://en.wikipedia.org/wiki/Medina_Azahara