Just learnt from mexico in November
Take US dollars to tip on the resort. $5 the first day to your room staff, $1 a day after gets you taken care of.
If you leave and go to a city/town, take pesos only, you'll lose your shirt negotiating otherwise. Anything you see that is mass manufactured (check the hotel store before you leave) can be neogotiated at least 50% off.
If you are told about one of a kind jewelry, make sure it looks (and is) hand made and not the 14k gold with perfect edges made in a factory somewhere.
Enjoy the sun