Permits for certain treks like the Inca Trail and Huayna Picchu sell out months in advance. Aim to secure your permits 6+ months before your travel dates to avoid availability issues.
Peru is a culinary mecca. Taking a cooking class in Lima or Cusco is a great way to learn about the ingredients used in Peruvian cuisine and to bring a new recipe home.
It's good to be prepared for the high altitude, but don’t let the fear of altitude sickness deter you from traveling to high altitude areas. Most people who get altitude sickness feel only mild symptoms that clear up with some time and rest.