Leon Viejo preserves the remains of Nicaragua's original colonial Leon, abandoned after earthquakes and volcanic pressure made the settlement untenable. The ruins are low and archaeological rather than grandly rebuilt, but their setting near Momotombo gives the site a strong sense of place.
It is one of the best historical excursions from modern Leon because it shows the city before the city moved. Go for the story and the landscape as much as for standing architecture, and combine it with views of Lake Managua or Momotombo if transport allows.