Cusco has some great sites, such as those around the Plaza de Armas, the grand Christian Churches and generally walking through the narrow cobbled streets. Plus there are obviously some of the more world-famous places to check out nearby such as the Sacred Valley (see Driving around the Sacred Valley), Hiking the 4 day / 3 night Inca Trail and Machu Picchu itself.
But I’d also really recommend taking half a day to see some of the smaller sites just to the North of Cusco, which come as a really pleasant surprise as they are walking distance from both Cusco and each other, and give you a bit of a mini adventure typically away from the crowds of the more famous spots.