I'll happily second Villavics recommendation. It was on my bucket list. Was there last year, well worth seeing the oldest city, imagining how it was then. BTW, was told it is situated and was important because it is there the crossing across the Andes is the shortest.
There is an less expensive company that takes a bus early. It had a silly stop on the way that only meant you arrived later to endure the heat. It is a short way. Leave early, arrive early to avoid the heat and the wait for the available tour guide. They have guides you need to use and they wait for the appropriate group size to start. Problem is I do not recall them having English guides.
Hey, afterwards continue on to the beach at MedioMundo and sunburn there.