Compared to the countries to the north (Mexico and Guatemala, for instance) and those in South America such as Peru, there are few archaeological remains of indigenous people in the Central American country of Costa Rica, where I lived from April 1976 until November 1980.
One exception is Guayabo, a Pre-Columbian site that was apparently occupied from about 1000 BC until 1400 AD, and then abandoned. Little is known about the people who lived at Guayabo, but it is believed to have been home to a population of more than 2000.
Guayabo National Monument lies about 18 km (and about 35 minutes) northeast of Turrialba in the Province of Cartago, and east of the capital city of San José, on the southeast slopes of Volcán Turrialba (that has been explosively active for the past few years).

Looking north to the summit of Volcán Turrialba from CATIE where I lived in Turrialba from 1976-1980.
In January 1980 when Steph and I (and a very young Hannah) visited Guayabo, it took about two hours each way from Turrialba, in a 4×4 vehicle. Obviously, in the intervening years, the roads have improved (map).
It is Costa Rica’s largest archaeological monument, covering more than 200 hectares. More has been uncovered since we visited in 1980. The various structures include mounds, staircases, roads, open and closed aqueducts, water tanks, tombs, petroglyphs, monoliths and sculptures. Some of its features show Mesoamerican influences, and others from South America, not surprising given Costa Rica’s location on the land bridge between North and South America

Carlos Humberto Aguilar
Artefacts from Guayabo had been studied in the late 19th century, but somewhat dismissed as insignificant. It took until 1968, when University of Costa Rica archaeology professor Carlos Aguilar Piedra (d. 2008) realised Guayabo’s true significance and excavations began.
More recent photo and artists impressions of the settlement can be seen in this post from the Two Weeks in Costa Rica blog.