Multiple swimsuits.
Mexico is HUMID. Simply hanging a suit over the balcony to dry overnight won't do the trick. Stock up on swimsuits before jetting down south.
Bug spray.
If you plan to do any cool excursions off-resort (which you so totally should!), chances are you might be in an area where the mosquitoes are a-buzzin'. Don't risk getting bit by a tropical bug. Not worth it.
Hand sanitizer. *Especially* since water off-resort isn't guaranteed to be clean if it's not pre-bottled. And if you're off-resort, always have bottled water with you!
Nice "Resort Wear."
A lot of resorts have restaurants on site that are already included with your stay. They also have dress codes...that they enforce. Better safe than sorry!
What's "Resort Wear?"
– For the girls:Longer maxi dresses, wedges, dresses or skirts that end right above your knee or below.
– For the guys: Collared shirts (they can be short sleeve), pants/shorts that come below the knee.
A Spanish dictionary.
Or bring along an index card, or a few words on your phone! Try to learn a few basic greetings, how to ask for things, say thank you, etc. It's a nicety that can go a long way and shows appreciation for another culture!