Look up "laid-back beach town" in the dictionary, and you just might find a picture of Monterrico, the only town in the country with such a vibe. Don't expect Puerto Vallarta, but Monterrico is a wonderful little place with a growing selection of lodgings, and even the upscale hotels aren't too pricey. Weekends are usually full here—you can be here from Guatemala City in just over two hours, and a huge number of the capital's residents do exactly that—but come during the week and you'll likely have the place to yourself.
Drawbacks include black volcanic sand that is very hot on bare feet at midday (wear sandals), and a wicked undertow that can pull you out to sea before you realize what is happening. (Lifeguards watch over a small section of beach on weekends only. Exercise extreme caution, and never swim alone, good advice anywhere, but especially here.)