In the main plaza at Cusco. The town is the central kick-off point for, basically, all tourists in Peru including all the backpackers preparing to undertake the Inca Trail to Machu Picchu. Later in the trip I spent two days here and have more pictures and stories about Cusco in that section.
The guide books recommend you leave Cusco as soon after arriving as possible, do your other touring and then return to Cusco as a way to lessen your chances of feeling altitude sickness since Cusco is at a higher elevation than the other tourist destinations including Machu Picchu.
I was quite energetic, read up-down-up-down where I constantly saw myself as a long toothed Mountain Goat, and was quite ok with the altitude, which I attribute to the coca leaf tea!