Córdoba

You cant get much more historically significant than this. The Jesuits were responsible for founding the first university in Cordoba and a library to go with is as well as building churches. The whole complex is a UNESCO heritage site but it is still the university today so you see it as it should be - full of students. Thanks to its status, it is very well preserved.
The Manzana Jesuitica is right in the centre of Cordoba so its very easy to get to. It costs nothing to go in and the courtyard is a great place to go and sit for a few minutes out of the sun and the throngs of people outside.
The church is beautiful inside and well worth a look.