One of the best place to visit in Mexico
Cancun : You can visit Cancun for the tropical weather. You can visit Cancun all over the year. You can avoid September to October because of little bit of rain and cloudy weather. 
Cancun has some splendid Resorts with All inclusive package . The beach is clean. 
You can stay either in Cancun or Playa Del Carman or The Cancun . Both has similar resorts mostly. You can visit Tulum if you like calm and nature associated with Beach.