Because of its rich and complex history Cuzco is a great city to visit; you will find fine examples of well preserved colonial architecture erected directly atop old Inca walls.
Out of the main attractions of the city be sure to visit:
Plaza de Armas, the main gathering place in the city. It's a great place to spend your time and feel the city vibe. Dominated by the old imposing Cathedral the square has also some good shops, restaurants and bars that any tourist will love.
Since the city is situated at around 3326m remember to take some time on arrival for acclimatization, spend a few hours taking a nap and take it easy until the nausea goes away; then you'll be ready to explore the streets of Cusco.