Wayna Picchu
Towering above the south end of Machu Picchu is this steep mountain, often the backdrop to many photos of the ruins. It looks a bit daunting from below, but while steep, it's not an unusually difficult ascent, and most reasonably fit persons shouldn't have a problem. Stone steps are laid along most of the path, and in the steeper sections steel cables provide a supporting handrail. That said, expect to be out of breath, and take care in the steeper portions, especially when wet, as it can become dangerous quickly. There's a tiny cave near the top that must be passed through, it is quite low and a rather tight squeeze. Take care at the peak, it can be somewhat precarious, and those afraid of heights may want to hang out just below. The entire walk is through beautiful landscape, and the views from the top are stunning, including birds eye views over the whole site. There's also a few ruins near the top. If visiting these ruins, you'll see a second way to start making your descent down the mountain, along some very steep and shallow steps.... these steps are a bit dangerous if wet, but the hike may be well worthwhile. This hike is one of your best bets for getting away from Machu Picchu and Wayna Picchu crowds. *after 2011* Tickets to Wayna Picchu had to be reserved beforehand - ask the tour company with which you are going to Machu Picchu to reserve this for you. you have to pay to climb Wayna Picchu. 2012: cost ~15$ Note: Only 400 people allowed per day to climb the mountain, split into two groups. Group one enters from 7 - 8 and is told to be back by 11.
If you got some energy in you, there's a few great hikes involving a bit of legwork. Do make sure that you've taken the time to acclimate to the elevation either in Cuzco or Aguas Calientes for a couple days before exterting yourself too much, especially on Wayna Picchu.
Moon Temple
If you have some time at hand, or long for a sparkle of solitude, you can also walk to the Moon Temple Templo de la Luna and the Great Cave Gran Caverne. It's a long walk and adventurous hike involving several ladders. Some may find that the sites aren't really rewarding, but unexpected wildlife can be seen wild spectacled bears have been reported. This hike is also quite interesting because partway through you leave behind the mountain terrain and enter a more conventional forest. The caves can be reached either by hiking down the trail from the peak of Waynapicchu which includes some semi-harrowing but fun near-vertical descents or by the split from the main Waynapicchu trail look for the sign that says Gran Carvern. Remember that it is much easier to descend from Waynapicchu than to ascend from these temples. Be sure to bring plenty of water and snacks for this long hike. The hike from the summit to the caves and back to the checkpoint takes about two more hours.