Last time that I went to Reus it was probably the least trouble of any Spanish airport with the GA parking within a very short walk of the terminal & as with all spanish airports the charges are cheap by british standards.
As said above all flights in spain require a flight plan, once you are in the system and they give you a transponder code then you stay with that code all flight and the ATC units keep a track on you.
To save your self a lot of messing about when fueling get yourself a BP fuel card, most of the fuel truck drivers don't like taking cash and are delighted if they see that you have a BP card as it cuts down on the paperwork for them and as a bonus you don't have to take wads of euros with you.
I have yet to go to a Spanish airport that will not take the BP card.