As Marbleskies said, stay hydrated & cool. When drinking lots of liquid, though, take advantages of restrooms whenever you can -- as a wise person once said, "Never give up a chance to pee." :-)
We've visited Spain twice this century, and because it's a big country with so much to see, our first trip was to just southern Spain (Granada, Sevilla, and villages in the Andalucia region), and our second was in more northerly areas (Madrid, Toledo, Segovia, Bilbao & Basque Country, Barcelona & Catalonya region). There's still lots to see that we haven't even gotten close to seeing, including the Galicia region. If you'll be able to return again, perhaps sticking to a somewhat smaller area would, in fact, cut down on the time needed for transportation. On the other hand, local airlines like Vueling can jet you around Spain quickly, but you deal with airport security and check-in time issues.
We only toured the Alhambra during the day, and used up lots of rolls of film (in 2002 we still had an old-school camera). One night, we had dinner at a Rick-Steves-recommended restaurant with an Alhambra view, which was really nice, all floodlit. We also toured on our own, using Rick's book and the on-site flyer as our guides. An evening tour might be great, and probably less crowded. However, after touring the palaces, we also enjoyed the Generalife gardens at the Alhambra, and those are probably best seen in the daylight -- maybe aren't even accessible at night?
In Granada, we also did a soak and massage at the Hamam baths described in Rick's book -- really a great experience, and made seeing the bath site at the Alhambra more interesting.