My only regret from my trip to Machu Picchu is that I didn't do the Inca trail. But it was with a class trip, so I didn't have a choice. You need a special permit to hike the mountain that specifies what time you're allowed to enter. I think our group had the earliest time (8a) and we tried to hike pretty fast which ended up being worth it because for at least 10 minutes at the peak there wasn't anybody else. Once other tourists started showing up, we had to climb over people and try not to fall off. Spending the day at Machu Picchu is like spending 8 hours on a stairmaster but where most of the stairs are only deep enough for the balls of your feet (unless you go sideways). Amazing view up Huayna Picchu, but make sure you've done a LOT of lunges before you go!