Exploring Spain (Madrid)

Hola!!

Guys I'm new in town and have been in Madrid for a little more than a month. It's has been a great experience so far at work and otherwise. I love the vibe of the city and how friendly and easy going people are over here. However I have a long holiday comping up and I want explore some more places in spain. I'm planning rent a car and drive around and cover as many places as I can. I start on the 11th of Aug and need to be back in Madrid on the 16th morning. The places that i wanted to go are Barcelona, Valencia, San Sebastián And Malaga. Although I know that it won't possible to cover all 4 in such a short duration. Therefore I need YOUR SUGGESTIONS for what can be given a miss and where I absolutely need to go,places to stay and things to do to enrich my Spanish experience.
Also I'm traveling with a friend of mine and can accommodate 2 more people in the car. So please feel free to join in if you can and/or help me figure out How I should do this.

Cheers!
Akshay