What a great choice! Are you planning on hiking the Inca Trail or just taking the train to see Machu Picchu?
The Nazca lines are a great must-see, and those can be arranged too, you view them from a plane. There are plenty of tours you can book. From what I hear, only spend a night or two in Lima once you fly in, I hear there isn't much to do. If you plan to hike the trail, get to the area a few days early (plenty of stuff to do -explore Sacred Valley, etc) so you can get used to the altitude because altitude sickness is a killer!!
Give us an idea of the stuff you'd like to do! A few tpunks on here have been to Peru (myself excluded... I had the plane ticket and all, and ended up not going - I still kick myself daily).