Spain is in fact a real car-country: public transport is perfectly arranged between cities and also inside the large cities, but between the villages and let alone the countryside or mountains it's a different story. Then own transportation is actually the best option. Of course you can also decide to fly and then rent a car at the airport, preferably via the internet. Some rental companies regularly have attractive rates, see the links below.Budget car rentals are FireFly, Goldcar and such discounters. Often economic to book direct, or via eg. www.carjet.com. The discount prices on the websites are somewhat misleading though: count on approximately 100,= euro fuel deposit they will charge you on pickup. (For that amount you can refuel twice here, but anyway).

But even with that, you're out cheaper than with Hertz, Sixt or Avis. Furthermore they will be quite pursuasive to sell you extra insurances, because included in the rental price is only a CWD with an own risk of about 650 to 800 euro. Whether or not you want to cover all risk is up to you (the additional insurance will probably be 150 euros extra or more), but be prepared they will put some pressure on you to pursuade you.
Furthermore, with regard to the "second driver" issue, if you would like to drive both you should pay attention on which companies offer the second driver  included. Most will charge a price per day extra, which puts a different light on the advertised rental rates ...