Whenever you travel to a foreign country it is always best to teach yourself some basic expressions in the language. Even a few phrases will endear you to the locals. When I went to Paris this summer, I could say about a dozen phrases in French. Smiling and saying a well-pronounced "Bonjour" seemed to work wonders. In spite of the stereotype that the Parisians are unfriendly, everyone that I encountered was very pleasant and helpful! I even had one person compliment me on my French! Ha!
If you travel to Mexico or any Spanish-speaking country, you should learn the greetings... "Buenos días" (Good morning), "Buenas tardes" (Good afternoon), and "Buenas noches" (Good evening). Know the golden phrases of politeness... "Por favor" (Please), "Gracias" (Thank you) and "De nada" (You're welcome). "Perdón" or, even better, "Disculpa" means "Pardon me".
Here are a couple other polite expressions which will make the natives think that you really know Spanish, or at least think that you are a very courteous person:
As you are leaving a restaurant, say to the people at the next table, "Buen provecho" or simply "Provecho." It is the equivalent of the French "Bon appétit." The diners will smile and say "Gracias."
Another handy expression is "Con permiso." It literally means "with permission", and is used in the way that we use "Excuse me" when getting up from the dinner table. If you are momentarily taking leave from someone, if you are trying to make your way through a crowd of people, or even if you are getting off the elevator, "con permiso" is the polite thing to say. The response to that (particularly in Mexico) is "Propio", which I frankly don't quite understand. "Propio" literally means "one's own" or "one's self", but that is what they say.
Of course, when you use your few phrases, people might start rattling off in Spanish to you. Then you can smile, and say, "No hablo mucho español."