Get some pesos at your local bank or at Travelex. You will have pesos when you arrive and the airport exchange rate is usually not very good.
Order a guide book for the Riviera Maya. Puerto Morelos will be included along with other towns along the corridor from Cancun to Tulum.
Get reservations to any parks or tours you want to take. Some offer early booking discounts and others sell out early
Bring a GPS that works in Mexico if you plan on driving. This Garmin model works in Mexico.
Make sure and bring enough of any prescription medicine to last thru your stay.
Let your debit/credit card provider know you when and for how long you will be in Mexico. Some debit/cards will not work in Mexico without prior notification.
Don't overpack clothes. Puerto Morelos and Cancun are very informal and a few beach clothes go a long way. Laundry service is available here if you need it.
Print out any map or directions to the hotel/condo/home you will be staying in.
Turn off the data for your cell phone if you do not have a plan that includes Mexico. Data charges will be very high and your smart phone will "ping" the Mexican cell towers during your entire trip. $2000 phone bills are not unheard of.
Bring sunglasses, bio-degradable sunscreen, hats and anything else that you will need in a tropical environment.
Keep a packing list of things you would like to bring on your trip. Add to it each time you think of something else and then check the list the day before your trip.
Bring dollars in both large and smaller denominations. You can use smaller bills for tips and exchange larger bills for pesos at a money exchange booth or Mexican bank. There is a manned money exchange booth off the town square in Puerto Morelos.
Bring a writing pen with you on your flight to fill out the immigration and customs forms before you land. It will save you time and put you ahead of the crowd. Bring extras if you want to make friends on the plane.