Cusco is a fascinating blend of old and new. It was once the capital of the Inca Empire, and colonial buildings now top Inca foundations flanked on all sides by the throngs of locals in colorful dress, visitors headed to Machu Picchu, and locals shopping in the daily markets.
If you arrive in Cusco with the intention of hopping on the train to Machu Picchu the next morning, you'll probably have time only to take a stroll through the Plaza de Armas and visit Qorikancha (the Temple of the Sun) and the Catedral. The city merits more exploration, however, at either the start or the end of your trip. Consider spending at least two days in Cusco, giving you time to acclimate to the altitude and to get to know this city of terracotta roofs and cobblestone streets. Note that churches and some restaurants close for a few hours in the middle of the day. Also, although some of the city's museums close on Sunday (such as the Qoricancha temple), most are open, albeit with shorter hours.
Cusco takes its newest role as tourist favorite in stride, and absorbs thousands of travelers with an ample supply of lodgings, restaurants, and services. That a polished infrastructure exists in such a remote, high-elevation locale is a pleasant surprise.