A birth certificate with a photo ID like a driver's license will suffice. However, some airlines will tell you that doesn't work. Don't waste your breath arguing with them. I did and all I got was upset and high blood pressure. If the airline tells you to get a notarized statement of citizenship, do it.
Any notary can create one, and the airlines have deals with notaries at some airports.
If you are flying into Mexico City, have no fear, despite the hype you may have seen on the news. You will not be anywhere except the airport, where the spa will pick you up. If you are staying in Mexico City, just don't take an unauthorized cab. That means take one from the airport that you have bought a ticket for. In the city, have your hotel call you one.