All i can say is make sure you understand a little Spanish - especially at the smaller fields, also make sure you ask them for the price of AVGAS BEFORE letting them fuel you up, and make sure that you understand the figure and if that includes all taxes. When I landed in Barcelona last year (Sabadell Airport) - I was told the price of fuel per litre to be just over €2.20, only to find out that they added an extra €1.20 (or just over) per litre of tax for being a non based aeroplane. I am fluent - but the fuel man who took over an hour to get out to me failed to mention this extra. For comparison - price at Biggin was £1.42 per litre, and €1.90 at La Baule (LFRE) - so do watch out! This made my fuel stop VERY expensive, I would have done better to fly across to France and stop ANYWHERE there for fuel.
I have since been told that a large number of airfields charge extra for fuel to non-based planes. Something which I find absolutely crazy as surely, in the interest of safety you would want people to fly with enough fuel, especially when flying further afield, rather than taking the least amount they can to save on costs!
I hear Granada is a good fuel stop (price wise), but cannot confirm this as I have no personal experience flying in.