As IO says, a flight plan is only required if the flight pass through some controlled airspace. If you can avoid controlled airspace, you don't need one.
Having said that, from what I remember it was fairly difficult to avoid controlled airspace! There is an awful lot of class D, and most airports seemed to be within class D.
I can't remember the transponder requirements as we had one, so that was'nt an issue for us.
One thing I would say is that the Spanish ICAO VFR charts are CRAP CRAP CARP! Airspace is extrememly difficult to read from them, and somthing that is best sorted on the ground! I'm not a fan of the Jeppesen VFR charts, but if I was going to Spain again, I'd get the Jeppesen ones in preference to the SPanish ICAO ones.
It's 18 months since I flew to Spain, so that info is 18 months old.
dp