Believe it or not, it's not easy from San Francisco. In the summer, there is one weekly charter from LA to Cancun, but it fills up months in advance. As well, we were leaving on a Tuesday. The only other option, and the option most other Bay Area people we met chose, was to fly from SFO to Guadalajara, then on the same plane to Mexico City, then change planes to Cancun. Not a short flight by any stretch of the imagination. Mexico City was a nice airport though - well organized and clean. All our flights (there and back) landed and took off on time.
We flew on Mexicana. It was embarassing that we had never heard about this airline, when it turned out they were celebrating their 75th anniversary! The only hitch we encountered was on the way back; my wife's vegetarian meal was grabbed by someone who hadn't requested one. Luckily the main meal was vegetarian anyways so no problem. There are no special meals on domestic flights.