Acclimatization in Peru is key. Even the first camps of many of the major climbs and trekking routes are high enough to make people seriously ill. As an example, if you are going to climb in the popular Quebrada Ishinca—launching pad for Ishinca, Urus, and Tocllaraju—the first day’s walk to the lodge leads to 4,350 meters.
The other major climbs are even worse: Pisco’s refuge is at 4,665 meters and Huascarán’s is at 4,700 meters! Most of the passes in the Cordillera Huayhuash [1] are between 4,500 and 5,000 meters, and the second day of the popular Santa Cruz trek is at 4,700 meters.
Instead of falling ill and ruining your trip, spend your first day in Huaraz [2] walking slowly up, through mountain villages, toward the Lazy Dog Inn or the Urbanización El Pinar. Another option from Huaraz is to hop aboard a combi heading east up over the Cordillera Negra [3] and get off at Callán Pass (4,225 meters). If you have a group, a taxi will only cost around US$30. From here you can walk or mountain bike back to town on a network of dirt trails. That night, sleep in Huaraz.
For the second night, many trekkers camp at their trailhead. Climbers, however, take advantage of the other good acclimatization possibilities near Huaraz [2]. A good second-day walk is Laguna Churup (4,485 meters), and a flat nearby option is Quebrada Quilcayhuanca. There is a place to camp at Pitec, the village near the Churup trailhead, at 3,850 meters.
Another second-day option, if you have your own transport, is a slightly longer drive up to the refuge in Quebrada Llaca (US$15 pp lodging, US$5 pp area admission). There is a lodge here and excellent camping spots. A knife edge moraine trail has mind-blowing views of Ranrapalca (6,162 meters) and the near-vertical south face of the Ocshapalca glacier (5,888 meters). The lodges on the outskirts of Huaraz [2] make good starting points.
Links:
[1] http://www.moon.com/destinations/peru/huaraz-and-the-cordillera-blanca/trekking-the-andes/cordillera-huayhuash
[2] http://www.moon.com/destinations/peru/huaraz-and-the-cordillera-blanca/huaraz
[3] http://www.moon.com/destinations/peru/huaraz-and-the-cordillera-blanca/trekking-the-andes/cordillera-negra