Spanish nudity laws
When I stayed at El Ports near Cartagena in 2014 not only was I naked on the beach, which is a known Naturist beach associated with the resort, I also went on walks, alone and with others, in the hills around there without any problems, despite being seen. We always dressed to go into town, of course. I believe the Spanish are relaxed about nudity, but I would suggest you don't push it. I'm not an expert on Spanish law, but I haven't heard of there being any laws against it, just use common sense.