Nearly every trip to Peru will include some time in Cusco. This is where many people come to before setting out on their way to Machu Picchu as well as a place to acclimatize to the altitude. If you're headed to Cusco, don't just treat it as a stop-over city on your way to Machu Picchu. The small city has much to offer!
It's very possible that you'll feel ill. Whatever you may have read or heard about altitude sickness is true -- for some people. At over 11,000 feet high, Cusco will try to break you. As soon as you step off of the plane in Cusco, you can definitely feel that the air is much, much thinner. The next few days can be pretty miserable, filled with piercing headaches, queasiness, loss of appetite, and general tiredness.