Just across the Peru-Bolivia border in Lake Titicaca, an 11 mile (1½ hr each way) boat trip from Copacabana.
The dwelling (or hiding) place of the Sun. I was somewhat constrained on time (buses to catch) so I didn’t take a tour but just went out on a private boat so maybe I missed all the amazing explanations and visits to other sites.
The island itslf has over 80 ruins, mostly Inca sites and mostly quite small. A brief visit limits what you get to see but I would wonder if the sites warrant a longer visit.
For other South American posts see South America Map.