My main points would be:-
(a) Someone on a previous exodus trip advises people to take pesos as difficult/not good rate to change. We followed this advice but found it to be wrong. It was easy to change money on our trip and US dollars could be used freely in Guatemala, Mexico and Belize. Some of our party used ATMs.
(b) Remember country entrance/exit fees. They add up though you are not always charged when leaving Guatemala.
(c) If you love textiles, buy them at Lake Atitlan (or Santiago Atitlan/San Marcos La Laguna). You just dont see as good/as great a variety elsewhere on the trip. There is a school shop near where you get the boat to Santiago Atitlan that sells lovely things in aid of the children's education
(d) Aquashoes useful as beaches quite stony. Quite a variation in temperature so need warm jumpers for altitude/night but light clothes for day. Binoculars a must for bird life and sea scanning for manatees.
(e) Vegetarian options can be a bit limited and traveller's diarrohea can hit hard even when taking care with handwashing/sanigel or equivalent!