Some points which might be of use to someone planning a visit to San Agustin and deciding how to visit its 3 locations (Based on a visit in Nov 2019)
a. Alto de los Idolos is closed for renovations until (at least?) Aug 2020. Worth double checking I guess if you arrive before this!
b. San Agustin is the main park and, with its museum, could take a full morning or longer.
c. It consists of 5 authentic “in situ” elements (3 “Mesitas”, 1 “Fuente” and 1 “Alto”) all situated within a fenced and “ticketed” area. There are paths (mainly paved) between them. Only the final “Alto” requires a short climb
d. There are 3 “Mesitas” (“A”, “B” and “C”). These are areas levelled by the builders of the San Agustin tombs to provide flat areas for the location of several groups of tombs. The tombs consist of “dolmen” which would have been covered by earth and have had guarding statues at their entrances.
e. We visited in the sequence B, C, A. I felt that there was a benefit on reaching B as soon as possible in the morning. All the tombs and stones are covered by modern protective structures and, as the sun climbs, these cast dark shadows across the stones making photography difficult (the worst is when the shadows cross half way up a statue!) The Triangle and the Eagle were fully clear when we reached them (on the other hand we had to postpone the Museum until the end!!) – see photo. This web page in Spanish has a map and pictures of the main statues to be seen at each Mesita as well as information on other aspects and the 2 other inscribed sites 
f. The “Fuente de Lavapatas” is an interesting example of San Agustin culture’s water management and its role in their beliefs and ceremonies. It consists of a waterfall over carved natural rock with the carvings representing creatures and mythological beings together with hollowed baths – presumably for rituals. A bridge has been built which makes it easier to see the falls from the bottom and there is a good plan/explanation of the carvings
g. The “Alto de Lavapatas” provides a view over the San Agustin area and has a few Statues brought together under a cover.
h. The “Bosque de las Estatuas” consists of a path through a wooded valley (still within the fenced area) in which have been placed some 39 San Agustin statues removed from other locations in the region. The above link provides photos of them. Some of them have “represented” Colombia at International fairs such as Osaka and Brussels. This is in effect an "open air museum" of course rather than strictly a part of the authentic site! But still worth seeing. The statues have signs showing a "given" name and period - but nothing else.
i. The Museum is a nice facility which provides a good introduction to the styles and iconography of the statues both via information boards and by containing many more “San Agustin” statues. The “problem” with San Agustin is how to appreciate and understand all the different statues being seen (akin to the Tierradentro problem of how to appreciate all the different but rather similar, Hypogea!!). It will become very easy to move on without properly appreciating them. We had an English speaking guide and he was able to point out features to look for on items which we would have missed/dismissed as “just another statue”. I didn’t see a guide book which could do the same. The explanatory signage at the site is not wonderful - each Meseta has a notice board with a plan and a short bilingual explanation of the total scene (period etc) but not at a more detailed statue level. Individual statues sometimes have a "given" name and period.
j. Alto de las Piedras is another group of tombs ("3 to 4 mounds") set on a hill beyond the town of Isnos. If you are trying to fit San Agustin into a single day you will have to decide whether to invest it all at the main park (probably a bit too long?) or whether to get out to Las Piedras in the afternoon (possibly a surfeit of statues??) They have their specialisms including some remaining colouring but how much they really add….? One, we were told, was the source of the wonderful "Winged Fish ("Pez Alado") which you will see at Bogota's Museo de Oro. Surprising, as the notice boards (at the mound level here) indicate that the tombs had all been robbed before excavation. 
k. We had 1.5 days at San Agustin and also visited some other nearby archaeological sites on the extra half day (together with the Magdalena gorge and a rather nice visit to a Campesino home) but you might consider taking them in if you have time. In my view only La Chaquira might be worth the effort both for its San Agustin statue carved into the rock and for the views over the Magdalena.