Great country to fly in. Here's what you need:
In theory you do not need a flightplan. In practice you do, as any flight from/to/through CAS requires one. There is not a lot of uncontrolled airspace around in Spain. Hence, you also need a radio and transponder.
Don't worry about CAS transits, just call up the controller before entering. On most long-distance flights you will be handed over from one controller to the next anyway.
You do NOT need slots at Spanish airports. This was only for Valencia during the Americas Cup. You DO however, need to check the NOTAMs as some airports (e.g. Malaga, Ibiza) limit and/or prohibit VFR traffic at certain times of the year and/or certain days of the week. Check and make sure you check older ones also, some of this info can be buried in a months-old NOTAM. Some airports (e.g. Badajoz) are mil/civ and have special procedures in place.
Radio is generally done in English, although a basic command of Spanish phraseology definitely helps, if only for situational awareness. Definitely worth brushing up on if you intend to fly into smaller fields. I think I've got a PDF with the most important phrases somewhere, PM me if you want it.
Landing fees are dirt cheap and are the same at all AENA airports, i.e. all the bigger ones. Fuel is generally not a problem, neither is (in my experience) parking. Exception here again being Valencia, where they force you to park miles from the fuel station and terminal. You then need to get ground handling and pay them EUR 60.- to get you back to the flight plan office.....
Charts: forget the Spanish ones, they are useless, get the Jeppesen.
International flights: Arriving from/departing to a Schengen country you can use any airfield. Not so going to Morocco (or Gibraltar, if you want to 'enjoy' UK-style landing fees). In this case you will need to use a customs field. Morocco usually likes a VFR flight plan filed with 24 hour notice, although I don't think this is a legal requirement. If you intend to fly to Portugal, you will need to cross the border at certain points (most, if not all are intersections) , this is a Portuguese requirement.
HTH