There's a lot to do in Spain because it is a big country. First, I will tell you what you should visit when you're in the capital: Madrid.
must see: royal palace, prado museum, reina sofia museum, museo thyssen, el escorial, valle de los caïdos, plaza mayor, puerto del sol and plaza de cibeles
Parc Guell, casa batlo, casa mila la pedrera, Montjuich, Montserrat, Sagrada Familia and Camp Nou.
Valencia: you should eat Paella (mmmm), see why it's called the city of art and science and see the Fallas (self-made animals/people from cardboard which they burn at the end of the event)
Andalusia: known for it's beaches and the events e.g. Semana Santa, la feria, cherry, ...
Sevilla: plaza de toros, cathedral with Geralda and torro del oro
Cordoba (Andalusia): mezquita
see you soon