If you come to Cancun for the Ironman 70.3, I recommend staying through Monday. You have to take time to enjoy the sights and to recover -- and what's a better way than to sit out on the beach and just relax.
I could not get enough of the beautiful crystal-clear and choppy Caribbean water. Here I'm thinking of next year's competition.
Surf is up and I'm all alone first thing in the morning (In Cancun you can start swimming at 0630 and the sun is already hot by 0700'ish)
I'm working hard with my MacBook on the beach outside the Condesa Fiesta Americana. The hotel provides WiFi that is as fast as a jetski, even out on the beach -- but at a steep price, and worth every penny.