This is all based on my personal experiences while traveling, and mostly through Eastern Europe in the late '80s.
1: Americans are loud obnoxious pricks.
We are. If you're a normally vociferous person (like me), everyone will hear you coming. Chatting is fine, but keeping it barely audible is best. Think museum-level voices, but everywhere. If you hear someone speaking English but can't see them, they're probably a block away and American. Or hooligans.
2: Stay away from hooligans.
Seriously. You'll just end up dead, broken, or with more knowledge of the Spanish penal system than you'd like.
3: Tourists attract pickpockets.
This applies anywhere you travel. The best thing you can do is have good situational awareness, and keep your important documents in the safest pockets you have.
4: Bring toilet paper.
This is more of an Eastern European recommendation because those commies sure don't like their cornholes. Most of their tp could double as sandpaper. You may find yourself using a public toilet, wondering how did I get here without any toilet paper, and then you'll be sorry.
5: Bring cash.
Credit cards aren't as useful in Europe. This is changing, but slowly. My burser on my last trip only paid for hotel rooms and some train tickets with a credit card, everything else was cash. You can get foreign currency from your bank if you give them enough notice, you can usually get a decent rate, or a better rate than you'll get from a cambio overseas.
6: McDonald's is worse in Europe.
Ok, so the last time I went to a McD's in another country was Turkey in 1987, but still it was horrible. Fucking coffee flavored milkshakes instead of chocolate. No wonder they lost the war.
7: If your flight lands in the morning, stay awake until night.
Best way to get over jet-lag, no matter what hypochondriac punkbitches say about melatonin or seratonin pills.
8: Bring American condoms.
You know your favorite brand, you trust it, so why use some commie condom from a former Soviet republic? Those things probably wouldn't stop a cold, and you wouldn't know the good local brand anyway.
Good enough for Eurotrip, good enough for you.
10: A few words about swimsuits.
It's Europe, you don't need a top, baby.