The Machu Picchu trek was amazing. Not only were the views stunning (getting even more beautiful with each day), but our group was incredibly fun.
Our second and third day were the toughest. The second day we hiked 1200 meters up over the course of 4 hours. The third day, though it was very hard, was a lot of fun and had the best views along the track. On the way we switched the already beautiful Peruvian mountains with the Peruvian Amazon. Making for an incredible atmosphere trekking in the mountains with jungle all around. The last part on the third day, called the "Gringo Killer" was incredibly tough but a lot of fun. Running down the old Inka tracks and through the jungle highlands.
I can only recommend the Inka trek for everyone who wants to see Machu Picchu. Don't just take a train ride up for a one day experience.