Guatemala hasn't inscribed any WHS since 1981, and their tentative list hardly gets updated. Almost all 2002 documentation is rubbish yet one of these, The Mayan-Olmecan Encounter (hereafter the M-O-E), had been resubmitted as Tak'alik Ab'aj National Park with an excellent description. I inquired if Tak’alik Ab’aj is the only site for the M-O-E and while technically not true it is the only touristy place, accessible place, "good" place to see the mix-up of the cultures.
First off, this is a bit far from the usual places in Guatemala. Me and my BFF took a road trip to the Fuentes Georginas hot springs (come very early if you want to go there it fills up fast!) and continued south to the park and ended up at the coast from where you can easily get back to Antigua - something I would recommend as itinerary. The road is pretty good, all paved and just the usual annoying road bump madness.
Second, do NOT expect Tikal, Calakmul, Chichen Itza or anything that grand. Most places here are in ruins. There is one set of grassy stairs that lead to what must have been a grande pyramid but it is just a grassy hill now. All other "highlights" are basically dig holes for carvings - and not very interesting ones for tourists to look at. Work seems to be ongoing. What you do see is unique, that's for sure, because the M-O-E is two cultures merging their architecture and starting out with mainly Olmec culture and showing more interest in the Mayan one later. Olmec is also super underrepresented on the WH list so it is due for something.
There are several paths you can follow as otherwise this place is complicated to figure out where what is. It also takes you past some kind of mini zoo of poor animals in cages - so that was maybe a little surprising. You can also choose to hire a guide but we never do that.
Basically a shoo-in if it ever gets nominated but that could take decades.