I know very little about Spain, I only got to visit Barcelona and Madrid. Nonetheless, here's my 2 cents.

Barcelona

Beautiful touristic city. A lot of young French people go there in Summer to party. An hour up North is Lorret De Mar, a smaller city, even bigger on parties.

Madrid

The capital can be visit in 4 days tops. It's a pretty city but there's not much to see, it's more about the atmosphere, the Spanish way of life, tapas, flamenco, etc. I recommend:

• Walk the calle Grand Via
• El Retiro Park: don't miss the Palacio de Velázquez, the monument to Alfonso XII and Madrid's Oldest Tree.
• Nearby is the Real Jardín Botánico. Pay only 4 euros to get in (guarden only, 2 extra euros to access the exhibitions)
• Puerta de Alcalá
• Cibeles Fountain
• Prado museum (no photos allowed!). Everyday, the last two hours before closing are free for everybody on most days.
• Plaza de Santa Ana
• Almudena Cathedral
• Royal Palace of Madrid (no photos allowed in most rooms!). Again, last two hours are free most days for EU and Latin American citizens.
• Puerta del Sol
• Plaza Mayor
• Mercado De San Miguel

Good to know: taxis are slightly more expensive than Uber. Average price I have seen for Uber from the airport to the city center is 25 euros, whereas I paid 30 euros for a taxi (it's also the maximum price by law).

Also, I searched for hours for a comic book store that sells the latest English comics, I could not find any. I found a few that sold months old English comics though (Red Flack Comics was one of them).

Last but not least, I would advise to visit Madrid in late May, as the weather is perfect, not too hot and not too cold (25 degrees on average in the afternoon), and the crowds of tourists are not there yet.

That's it for now!