The Place
Cusco was the political and religious center of the Inca Empire for roughly 300 years, and unlike Machu Picchu, it never got the luxury of being forgotten — the Spanish took it, built over it, and lived in it continuously ever since. The result is a city where you can stand in a colonial-era church and see, at the base of its walls, the original Inca stonework the Spanish built directly on top of rather than tearing down.
Sacsayhuamán, just above the city, is the clearest example of what makes Inca stonework genuinely remarkable: some of its blocks weigh over 100 tons and were cut and fitted with a precision — no mortar, joints so tight a knife blade won't fit between them — that still isn't fully explained by the tools archaeologists know the Inca had access to.
Two full days here — after Machu Picchu, before the flight back to Lima — is enough to actually walk the city rather than pass through it: San Pedro market for produce and street food that has nothing to do with tourism, the Plaza de Armas at night when the cathedral and the Jesuit church are lit up, and a chocolate-and-pisco museum that's a genuinely good primer on two things Peru does better than almost anywhere.