The Inca Trail winds through the Andes Mountains, the classic route being a 4-5 day trek from Cuzco to Macchu Pichu. Concerns for overuse and erosion have resulted in regulation of the number of hikers that can hike the trail each day. Only a limited number of guides are allowed to take travelers on the trail as well. The trail is closed in February for cleaning, as well.

For all these reasons, reservations are a must and high season fills up quickly.

More information on Wikipedia.