I wouldnt stay in Cancun full of infurating Americans., Playa del carmen was great, there are plenty of little bungalows to stay in. In Guatemala there are also little hotels everywhere, depending on where you are going- I wouldn't stop in Guatemala city but head for Antigua and Lago Atitlan-look in Lonely planet for some good recommendations for Accomodation. Belize is pricey and & again a lot of Americans, head for beach and dont spend too much time in Belize city.