The mountains of Peru are at a high altitude. Many people come to Peru to experience the natural wonders of the Andes, and it is important to stay hydrated and rest when at high altitudes, especially in the first couple of days of adjustment. When you arrive in Cusco take about 1 day to get adjusted to altitude make sure that you are watching what you are eating, drinking, and alcohol consumption.
At Machu Pichu, the restrooms are located outside the entrance to the site and no re-entry is allowed. That means once you enter, you will likely not have access to a bathroom during the 3-4 hour visit.
Norms around personal space are different in Peru than they are in the US. Typically Peruvians will stand much closer to each other than Americans when having a conversation.
In terms of clothing, in the more traditional, indigenous regions of Peru, you should dress conservatively, long (knee high) shorts or pants and tee shirt or long sleeved shirt. Also, practicality here is important, the sun is extremely strong so generally people try to protect themselves from it. You should follow their lead.