For future ref:

Noting that there are now 2 shuttle companies that connect Flores with Palenque Mexico, so an overland trip Cancun>>Belize>>northern Guatemala>>Palenque>>Oaxaca is now possible if that mostly lowland experience interests you.

​​​​​​​I am visiting Belize in December and rather stumped finding a decent beach, something that’s not just a pathetic scrap of sand.