I'm not sure about the things specific to Mexico, but here are some things you shouldn't go without!
Bandages, both gauze and ACE bandages(for sprains and strains)
Sunscreen
Pain-killers (ibuprofen, etc.)
Tweezers
Anti-bacterial cream
Diarrhea relief
Adhesive bandages
Foot care items: athletic tape, blister bandages, "moleskin" patches, etc.
Anything else you tend to need: asthma inhaler, for instance.
Also:
Always be willing to turn back if your trip is threatened by bad weather, injury, or the like. The journey is the point, not the destination. Bad things can happen if bad decisions are made out of stubbornness.
Carry spares wherever possible: things break and get lost; bulbs burn out and boot laces break.
You can buy foldable stoves that burn solid fuel bars and fit into a pocket: a marvellous saviour when everything is too wet to burn and you are in dire need of a hot beverage.
Rope always finds a purpose. Polypropylene rope is light and very strong and can be used for washing lines; storm-setting your tent; hauling people out of holes in the ground; crossing rivers; dragging your rucksack up a cliff face and hanging your kit out of the reach of animals.
Whenever possible, start pitching camp at least an hour before sunset. Setting up camp at dark is much more difficult. *Antipersperant rubbed onto your feet can help avoid blisters.