Religion plays a big role in Spanish culture. You can see many churches in the places you visit, often very ornamented and majestic. Spain is also home of the most famous church in the world: Sagrada Familia. Perhaps not so many young people consider themselves to be religious and the percentage of people attending masses every week is descending but religion still has a big role in its culture. Especially on the important holidays, there are many traditions they still follow and many of their holidays have a religious origin. Spain has a long and rich religious history. The Santiago de Compostela trail for pilgrims, takes place through the northern part of Spain. It is very famous and many people are doing it every year, sleeping in ''albergues'' - hostels especially for pilgrims.
One of the most important aspects of Spanish life is the family; no celebration would be complete without an extended gathering, although this is more common away from the busy cities where modern life takes its toll. Even so, the elderly are respected, and it’s not uncommon to have older relatives being cared for in the family home. Likewise, children are absolutely adored, and included in everything.
Spain is slow. That is to say, when you arrive, you'll have to tone it down a bit. They walk slower. They eat slower. Their bureaucracy is more intricate and fickle, and slower. Things are slower here. Have you just asked for the check? Expect to wait another 20 minutes before it arrives. Do you want to pay by card? Add another 5 minutes to your wait time. Take a deep breath, and meditate on the beauty of the cobblestones while you wait. They're not trying to be rude, they're just in no hurry...
Respect the line. Spanish people line-up for the bus, to take the train, to snag a free something or another in a plaza, for church festivals, etc.
If you are planning to indulge in any topless sunbathing, consider local feelings first, and try to stick to beaches where people are already doing it. You also need to make sure you are properly covered if you enter a church; shorts and sleeveless tops should be avoided. Same goes for wearing bikinis outside of the beach. Don't do it, it might be considered disrespectful.