AMexico: Day of the Dead festival
Can you be more specific. Do you want cultural, sea and sand? I have been to almost every corner of Mexico. Look at Playa del Carmen, Isla Mujeres, and Ixtapa Zihuatanejo. Merida is...
ACancun: Tulum or Chichen Itza - only have time for one, but which one?
Tulumis the closest. Go into the city by buss or colectivo and get a cab to take you to the beach south of thre ruins. The beach is the best in the world and you can walk north to the ruins.