Basically you want to be prepared to be in warm or sunny, hot, humid weather during the day and cool sometimes humid weather in the evening. Additionally, it usually rains once a day, in the middle of the day for an hour or so. 90+% of the time you will be in shorts and a t-shirt or short sleeved shirt with flip flops. But it can get muddy after a rain so have a pair of hiking boots as well.
I always bring little gifts (like Cabot cheese and Vermont syrup) but do not feel you need to.
Following is a non-exhaustive list of things to consider:
Laptop computer/ipad/smart phone
Hiking shoes (lightweight and breathable)
Lightweight rain jacket
Lightweight long sleeve fleece, for the occasional cool evening
Lightweight long pants
Bandana or scarf for multiple uses (wash cloth, napkin, sun screen, etc…)
Paper copy of your passport, in case you lose your original. I save a scanned copy on my computer as well.
Spanish – English Dictionary
Small first aid kit