We aren't quite sure what the ancient city of Machu Picchu, in Peru, was built for, but as a UNESCO World Heritage site, it's recognized as an important archeological resource, as well as one puzzle left behind by the Incas (the Rubik's Cube is the other).
Situated on a ridge 2,000 feet (609.6 meters) above the Urubamba River, Machu Picchu is made up of over 700 terraces. Though researchers have not definitively proven the purpose of Machu Picchu -- and the Incas didn't have a written language, so they didn't leave any records -- it appears to be the center of a network of trails and other sites. Many of the sites align with astrological events. Machu Picchu was built without engineering tools we take for granted, like wheels, which makes it even more impressive.
For a true Machu Picchu adventure, you can hike to the site on the Inca trail, which takes about four days. You must hike the trail with an organized group. While that may bug introverts, a knowledgeable guide will point out key features of the trail, share information about the Incas, offer a window into modern Peruvian culture and may even have extra band-aids for your blisters.