Don't use the word "indio", although it's Spanish. It was used by Spanish conquerors. The politically correct way of speaking is "el indígena" or "la indígena" - although very close people inside a circle of friends can get away with it. Another word to be careful with is chola/cholo or cholita, meaning indígena. This may be used affectionately among indigenous people (it'a very common appellation for a child, for instance) but is offensive coming from an outsider.

Even if you have about 20 "No drugs" T-shirts at home, accept that especially people from the country side chew coca leaves. See it as a part of the culture with social and ritual components. And keep in mind: Coca leaves are not cocaine and they are legal. You can try them to experience the culture. If you don't like to chew them, try a mate de hojas de coca. Also quite effective against altitude sickness.

Officially, most of the Peruvians are Roman Catholic, but especially on the country-side, the ancient pre-Hispanic religiosity is still alive. Respect that when visiting temple ruins or other ritual places and behave as it were a church.