Hello!!! I was reading the comments about Cancun, but the most of then are a little wrong, well, all depends that you wanna do! Because Cancun is full of tourists, the best option is Playa del Carmen near Cancun Airport.
On the other hand Xelha and Xcaret are beautiful but if you want, you can go to other "cenotes" like Cristalino near Akumal or "cenote Azul", so if you wanna surf you must go to Puerto Escondido in Oaxaca.