Six days in Spain (excluding Madrid)! You could spend a whole lifetime and not see everything, so you will have to be very selective. If your plan is to do the undetermined bit first, then go to Madrid, then I would think the best thing would be to fly in to wherever and out of Madrid. That will save you one trip from Madrid to whichever part of the country you're going to first, but of course it all depends on where you are coming from and which airports are available to you.
First, I think you have to decide what interests you about Spain. Do you want to spend your time in cities, viewing historic sites, visiting villages and mountains, seeing coastal scenery, or what?
If cities are your thing, unlike Casaianda, I think Barcelona is one of the best cities to visit anywhere. You could spend two or three days there then move on to the Costa Brava for some pretty villages and a lovely rugged coastline. If time permits, it might be possible to make it up to the Pyrenees too.
Another city choice would be to cruise around Granada, Seville and Cordoba. Fly to one of those airports (or Malaga), hire a car and do the tour. As well as the cities, you would get to see the wonderful mountains and pueblos blancos of Andalucia, though you won't have much time to really get off the beaten track. Just keep away from the motorways though and you can't fail to touch on some great places.
For a really nice trip that would soak up the real character of Andalucia, start at Malaga (for convenience of flights) then drive up into the mountains via Casares and stop in Gaucin. The next day you could move on to Ronda. Walk down into the ravine to gaze up at the amazing bridge. Take time out to to take the narrow-gauge railway trip. After that, make you way up to Granada, to visit the Alhambra of course (be sure to book in advance) and also the Albayzin. Finally, make your way back down to the coast via any route you like (they're all sensationally scenic), visit the caves in Nerja and back to Malaga. Six days is not generous but it's possible.
[ Edit: Sorry, no promos please. ]