In Barcelona, definitely go to Parque Guell, the Sagrada Familia, the Picasso Museum (one of my favorite art museums in the world...and I've been to MANY as the daughter of two artists), and of course, the beach. Definitely go to the Alhambra (amazing!!!!) in Granada, when you're down in Andalucia. In Sevilla, check out the huge mosque (soak up all you can about Spain's rich history of the Christians/Jews/Muslims while you're there...they call the interaction for so many years the "convivencia,"...it's really fascinating). Along those same lines, if you can while you're down in Andalucia, go to Cordoba and see the Mosque there - it's pretty cool (http://en.wikipedia.org/wiki/
In Toledo, wander and get lost around the little streets - it's a fabulous little ancient town. In Madrid, check out the Palacio Real, go to Plaza Santa Ana and have some tapas outside in the big square, go shopping at El Corte Ingles (Spain's biggest department store) in Plaza del Sol (the center of the city), relax outside and walk around Retiro Park (maybe rent one of the paddle boats and go out on the pond in the middle?), go to the Prado Museum, and go out ALL night! If there's a Real Madrid soccer game while you're there, that could be really fun too. Spaniards love their futbol. For nightlife, I suggest Palacio Gaviria, it's this really cool huge club in what used to be a palace in the middle of Madrid. Just beware --- drinks are $$ there!