In Colombia and Peru avgas is around $6 - $7 / gal nowadays.
Cartagena - SKCG: fair amount of General Aviation. Lots of hotels, beautiful downtown area to visit.
Cali - SKCL: I believe going south this is the last airport in Colombia that is officially a port of entry. If you want to leave the country via Tumaco or other more southern airport you need special permission I believe. Cali has good service, a few GA aircraft, avgas. Airport hotel has 12 rooms and is regularly sold out but more hotels in Cali or Palmira.
Guayaquil - SEGU: Lots of General Aviation traffic, easy in and out. During normal business hours there's a GA facility which functions similar to an FBO. Airport is in the city, hotels nearby.
Peru: Only Trujillo (SPRU), Lima (SPIM) and Pisco (SPSO) officially have avgas. Avoid SPIM if you can because it's quite busy with airline traffic and really has no General Aviation facilities. Trujillo and Pisco are good GA airports. If you do want to land near Lima try SPLX which is private (google Master of the Sky). SPLX has avgas but not sure if they sell to the public or for in-house use only.
I usually fly to Colombia via FL to Jamaica:
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