Spain packs an astonishing variety into one country: Gaudi's Barcelona, the museums and nightlife of Madrid, Moorish palaces in Andalusia, pintxos bars in San Sebastian and hundreds of beaches from the Costa Brava to the Canary Islands.
Life runs on a later clock here. Lunch starts around 2pm, dinner rarely before 9pm, and many shops still close for a mid-afternoon break outside the big cities. Adjust your rhythm and the country makes a lot more sense.
Spain is also several countries wearing one passport. Catalonia, the Basque Country, Andalusia and Galicia each have their own language, food and stubborn pride. Learn the local name for things — txakoli, not just 'white wine'; pintxos, not tapas — and you will be treated like someone who bothered to notice.
Do not try to do Barcelona, Madrid, Andalusia and an island in ten days. Pick a coast or a city-and-region pair, eat late, nap if you need to, and let the heat dictate the afternoon. The travelers who love Spain most are the ones who stopped treating it like a highlight reel.