Apart from the obvious stuff, don’t forget to carry the following items with you!
In november and december temperatures around Mexico are warm almost everywhere. The exceptions are the Mexico City, Oaxaca and San Cristobal areas. I’d advise that you always bring a sweater and a jacket with you if you plan to travel to these destinations. The coldest point, for us, was an evening in Oaxaca when the wind was blowing and the temperature was around 10°.
If it is true that Mexico’s climate is warm, the same cannot be said for the temperature inside night buses. They usually turn the AC way up so keep a sweater handy if you board a night bus because it can get very cold.
On the other hand, when sunny it’s very hot so bring something to cover your head with you. My recommendation would be to buy a cap there because they are cheap.
Always pack a swimsuit in your backpack when leaving for excursions and tours because you never know when it will get handy. With few exceptions I bathed almost every day in some pond, ocean, river etc.
In some places near and in the jungle there is the dengue virus so keep you mosquitos repellent in handy and pack some light fabric long-sleeved shirts as well as pants.
I brought with me 7 t-shirts, 7 underpants, 7 pair of socks for this trip planning on washing them whenever we had the opportunity. I think I could have made it with 5.
Bring a pair of water shoes always with you! I didn’t think of that and oftentimes during the drip I wish I had.