Machu Picchu.. You have to see it! That´s all I can really say about it. Pictures do no justice.
The Sacred Valley is also good.. make sure you´re in the mood to climb stairs because there are plenty.
We went through AAA Tours which was negotiated down to $185US (and $165US student) for both the Sacred Valley tour and Machu Picchu. Not sure if that´s average but it would pay to shop around as it seems that all the tour agencies use the same pool of guides anyway, so aside from the cheapest method of arranging everything yourself, you´d be best to find the cheapest tour you can.
