Cusco is a heck of a fun town and a great destination for a last minute holiday. Unlike many vacation destinations, there is always something going on in Cusco, so if you just randomly come into some free time, head for Cusco.
Sure, there are certain times of year when you should make an effort to be in Cusco, like during Inti Raymi for example, but even if you aren't in Cusco for some established festival, chances are there will be parades in the street anyway.
Cusco's just one of those fun places that draws a lot of traffic, and which the locals are very adept at dealing with travelers.
I happen to like it just for the streets, although you sometimes take your life into your hands when walking around there. Most of the city streets are designed for the passage of one man and one donkey walking side by side. Needless to say, this makes for a tight fit when somebody tries to drive a car down these roads.
Really, all roads within about a four mile radius of the Plaza de Armas should be closed to vehicular traffic. The way it stands, you should always make sure there is a doorway near by you can jump into in case a car comes speeding along. Honestly, there's a scene in the original Superman that reminds me of Cusco when a character stands on a recessed ledge as a subway train goes by (I always think of that scene when I'm traveling there).
But I'm exaggerating, for the most part the streets are so narrow that the traffic moves very slowly so there really isn't any danger. Also, it's the quaint, narrow layout of the city streets that give Cusco it's charm. So check it out! There's really no need to go to any of the museums or to Machu Picchu, you'll find plenty to do just on the Streets of Cusco--believe me!