What matters the most to you when planning for a trip?
I plan my trips to Peru around food. It is the most incredible cuisine in the world.
So, I make sure I have enough days to eat at least the following: Lomo Saltado con Tacu Tacu, Ceviche (x2 at least), Pollo a la Brasa, Churros from Manolos, Picarones, Anticuchos, Papa a la Huancaina, and Conchas al Parmesano.
This is just a starting point. Ideally I would include all of my other favorite Peruvian foods as well.