What I find most fascinating in Spain (besides the excellent wines), is the contrast of the bustling life in the cities versus the quiet life on the countryside and in the smaller cities. Sometimes, when you drive on the highway from one large city to another, you feel almost alone (due to the low amount of traffic) and I personally sense a kind of “Wild West” feeling.
As you probably know already, there is a lot to be seen everywhere, be it monuments, cathedrals, historic buildings, excellent restaurants, well-dressed people enjoying their life. Plus very good food, red wines, and alcoholic spirits. Spain is really worth many holidays.