A broad topic indeed!!!
Here are my two cents (of euro of course :P).
You could visit this places:
Madrid:
Museo del Prado
Palacio Real (get lost in the streets near Plaza de Oriente)
walk through Gran Vía
etc... (I'm sure someone can add tons of places)
El Escorial/San Lorenzo de El Escorial
Visit the Monasterio, it's worth the visit, and walk around
Barcelona
There's plenty things to see there (La Sagrada Familia is one of the best)
Mallorca, great!
Another coast places: Marbella, Benidorm,... But one of my favourite places: San José, in Almería.
Try San Sebastián in the Basque Country, beautiful.
Val d'Aran/Valle de Arán. It's wonderful, one of the best examples of the mountaineous Spain.
Santiago de Compostela (Galicia), if you want to try one of the beautifulest corners of Spain.
My suggestion is that you only do a basic plan (in which cities to stay every day), and then organize yourself on the fly.
Fernando