The greatest event in Cusco is without a doubt the Feast of the Sun or Inti Raymi. As I wanted to get the best place at Qoricancha I had to wake up very early to be there at 6 o´clock even though the Inca showed up at 9 am. The feast continued at the main square at 11am. In these two locations, you do not have to pay anything.
The third part of the feast held at Sacsayhuaman starts at around 2 o´clock and most people prefer to hike the hills in front of the esplanade of Chukipampa. Again, it is free but the piece of advice is to get those three scenarios two or three hours before otherwise you would barely see something. On the other hand, people who want a closer look to the show at Sacsayhuaman have to buy tickets at EMUFEC.