Basic Introduction
First please note there are no fashion police in Spain and that when you explore this country one will see many sorts of fashion being worn. The aim of this guide is to suggest what you should include in the limited space of your luggage, without dictating any hard and fast rules.
Spain is a modern country, but with many traditions of dress. Traditional/region dresses are usually worn at fiestas, and then not by everyone. Try and attend a fiesta or two to see just how diverse the traditional clothing was.
The typical visitor to Spain will be using only one medium sized suitcase, so it is advisable you take care packing clothes that will fit in one suitcase. Another thing to think about is that Spain does get all of the seasons of the year, meaning spring, summer, autumn/fall, and winter, so pack with layers in mind.
You may want to pack lightly and leave some space in your suitcase for souvenirs you may want to bring home.
There are hardly any self-service laundromats in Spain. However you can find tintorerías (dry cleaners) who will handle your normal laundry. Alternatively one can purchase liquid detergent at any supermarket. So you do not have to pack this. If you are staying in an apartment, it may have a machine to wash your clothes, but it may not have a clothes dryer.
It is better to bring clothing that does not need ironing, because you may not find an iron in your accommodation.
It is better to bring dark pants that will hide any stains.
Any bags should ideally have zippers, to reduce the chances of being pick pocketed.
The beach.
Away from the beach men can wear shorts and T shirts, with flip flops, if they want to. The same would also be ideal for women too.
Men and women should not wear swimming clothes at any restaurant, unless it is at a beach side restaurant.
A tip to women who want to go topless on the beach. The law permits topless attire at every beach. Some beaches are designated nudist beaches.
The Urban Environment
Ideally dress with layers in mind. Buildings can be hot and stuffy whilst the outside temperatures are low. Sometimes when visiting older buildings (like cathedrals) you will find them chilly compared with outside temperatures
Remember that some, but not all, places of worship have enforced dress rules. If these are applicable please ensure you abide by them.
For chilly days or nights, and for going out to restaurants, jersey/cardigan or lightweight jackets would be ideal attire.
Bring some sort of waterproof clothing, especially during spring, autumn/fall, and winter periods.
It should be noted that at a few hotels restaurants and clubs, if you are dressed inappropriately, will not let you in.
Note: one thing you may find at odds with is. Many (mainly older women) wear fur coats during winter. The attitudes towards wearing of fur in Spain is acceptable.
In addition you can see some photos taken on an average Spanish streets illustrating just how diverse dress standards are. HERE
So to sum up: Do not worry about trying to fit in with the Spanish. Relax and enjoy your vacation.