Reason enough to visit Barcelona is the never to be completed
Temple Expiatori de la Sagrada Familia. It was started in 1882 and they are still working on it today. If the Church was finished it would be a definite world wonder. I can't wait to go back when it is done. More....
The great thing about Madrid is the numerous day trips you can take into the country. We visited Toledo, the walled city of Avida, Segovia, The Valley of the Fallen plus many More... 
- Seville is your first chance to see an Alcazar plus visit what is described as the definition of Spanish lifestyle. It includes eating late, drinking strong drinks and don't forget the siesta More....
If you need a change of pace and a great tour of British colonial history then stop over for the day in the country of Gibraltar. More.... 
If you really want to get crazy hop on a boat and head to Morocco for the day. More... 
A trip to Spain would not be complete unless you visit Granada and the Alhambra. A day trip to Ronda isn't a bad idea either. More...