Machu Picchu was pretty awesome. How they were able to fuse the huge blocks together without the use mortar is rather impressive.
Go in a cooler time of year (even though it is high altitude it can get hot up there). Take a sweater that you can remove if you need to. Also go early in the day it can get crowded. It's amazing the difference between the first (1983) and the second time (2007) I went there. Dress conservatively and respect the people and their customs. Many locals really don't like their picture taken so ask first. If you are told not to go in a certain area (forbidden area) don't go! Eat local food it is wonderful! American food really is awful outside of North America. Remember that they too call themselves South Americans. We are North Americans. Lima has some beautiful architecture and cool museums. Your local hotel can offer you or tell you where to get site-seeing deals. Remember you are expected to bargain with the taxi driver and the local bus ride (collectivo) can be an adventure also if you have the time
