Pacbitun, a small site, 12 miles south of San Ignacio via Cristo Rey Rd, near San Antonio, seems to have been occupied continuously through most of Maya history, from 900 BC to AD 900. Today only lofty Plaza A has been uncovered and partially consolidated. Structures 1 and 2, on the east and west sides of the plaza, respectively, are worth a look. You may find archaeologists and researchers working onsite.