Oh, ancient Cuzco.
It's a must-see city.
And I would agree, it is a must see city: blue accents, epic plaza, lots of fun, narrow streets, people watching...
But caution: it also knows far too well that it's a must see city. Prepare yourself for people approaching you to sell a variety of shit and offer massages every two feet. One afternoon Walker and I made the mistake of sitting on some church steps in the main plaza, only to be swarmed. Eventually you harden your heart and develop tactics to turn people down even before they can open their mouths. It seriously wears on you.
I would definitely recommend touring every church in the plaza -they're all pretty epic inside. The Catholic church features a painting of the last supper with cuy being served on the table, an excellent example of how the Spanish tried to work Incan culture into conversion.
I was also amazed by the complete lack of being able to conceptualize the distant past. Most religious scenes depicted feature Christ, Mary, etc. wearing Spanish colonial style dress. So hilarious to picture biblical figures wandering through the desert in crushed velvet pants.
Stop to watch every street performance/saint celebration.
Also walk the steep trail through the city to peek into the Sacsayhuaman ruins (pronounced sexy woman - no joke) and go the extra mile up to the giant Jesus figurine - a la Rio de Janeiro and a standby in every South American city - where you can get an even better (and free!) view.