Adding on to Cheap Elite's suggestion, I might also just scan Starwood's list of category 4 hotels (which includes Mexico btw), see if anything piques your interest and do an individual search on that property.

Obviously your best bet would be Florida if you need a beach. California's waters would be too cold. If a beach isn't as important, some place like the Westin Mission Hills in Rancho Mirage or some of the Arizona resorts could also work.

And no matter how you define Mexico's location, the only thing that really matters is how the OP's frequent flyer program defines it. Sounds like the Caribbean and Hawaii require too many miles, don't know if that's also the case with Mexico.