Passport
Mexico requires only that your passport be valid for the duration of your stay. There is NO 6-month rule — but many airlines apply one anyway. Renew if you have less than 6 months left.
Return ticket
An onward or return ticket is technically required. Airlines check this at check-in. Mexican immigration rarely asks.
Proof of funds
Officially required but rarely asked. Have a credit card and a printout of a recent bank statement just in case.
Accommodation
You'll be asked for the address of your hotel on the FMM. Have it ready on your phone.
Insurance
Travel insurance is NOT a legal requirement to enter Mexico, but it is strongly recommended. Healthcare for travelers is private and cash-up-front.