Here is another one of those crazy low $220 American fares from San Francisco to Cancun for August peak humidity season. The catch is most itineraries require an overnight for US Airways flights in Phoenix outbound, inbound or both ways, so you have a chance to bake and sweat on the same trip.
There are six or seven dates with $220 fares and a few more dates in August and September with round trip fares under $300. I see 11 dates under $250 for August departures on Google Flights.
American Airlines San Francisco SFO – Phoenix PHX – Cancun CUN
$219.82 Wed Aug 12 – Wed Aug 19
4,822 flight miles American Airlines O class