The markets in Cusco are fabulous places to buy souvenirs, food to prepare at your hostel and full meals.
The most famous is San Pedro but I also love the San Blas Market, the Wanchaq Market and the Saturday market in the Plaza Túpac Amaru.
Markets are usually open from 6am to 5pm. They are great for breakfast and lunch but don’t plan on dinner at a market. See Eating at Markets.