AmyBEMT,
Unless this was a last minute trip, you should have purchased a passport.
Since my last name is different from my name on my birth certificate, should I take a copy of the marriage certificate indicating that change?
I would say yes. You should always be accurately documented.
I think you might need a US passport to get into Mexico, I am not sure,...
You don't need a passport to get into Mexico, as I've seen plenty of people using their drivers license and birth certificate while going to Cancun. I cringe at the thought of using my drivers license as documentation.
I personally would never travel internationally without a passport, even to Mexico.
Does anyone know if we can change money at the Ixtapa airport or can we change some at O'Hare before leaving?
Since you are flying from ORD on a domestic flight, I would think that there would be no money exchange counters. However, I never fly CO so I can't say for sure.
There may be one in IAH depending on your gate.