Socks in Sandals
No one would dream of wearing socks in sandals at home, so why do it when you're in Spain? The surest sign that you're "not from around these parts".
Other fashion no-no's include baseball caps when it isn't sunny and t-shirts that announce the city you come from.
It might seem hypocritical for Spaniards to frown on tourists for talking loudly when they speak at a high volume themselves, but remember that you're speaking in a foreign language - and this stands out more.
No one expects all foreigners to speak fluent Spanish, but flying up to a Spaniard and barking "Where is the train station?" without at least asking first (in Spanish) if they actually speak English is plain rude.
Oh, and no, speaking louder does not make you easier to understand!
Eating and Going Out at 'Un-Spanish' Times
The Spanish eat late and go out even later. If you don't follow suit you will (a) be eating alone and (b) be the subject of ridicule.
Do you want to advertise the fact that you are unfamiliar with the city and are the perfect prey for pickpockets and muggers? No? Then don't carry your map and travel guide for everyone to see. Stop in a café to check where you're going or, better still, print out pages from About.com and discretely refer to them instead!
If you're in Spain for two weeks, what's the rush with trying to get a tan? Spending all day on the beach on your first day without any sunscreen will leave you burnt to a crisp and in so much pain that you don't dare sit on the beach again. The burn quickly fades and by the time you are home, you're as white as when you started.
If you're traveling in summer, unless there's a storm coming, you have all the time in the world to get a tan, so take it easy!