Considered by many the most beautiful of the three valleys in the Four Sisters Mountain area, Haizi Valley's eponymous lakes and majestic views of the Four Sisters peaks make it hard to disagree. It's a steep walk for much of the way, so acclimatise in the other valleys first if possible. The entrance to the 19km-long valley is about 500m from the tourist centre, up an obvious staircase to the right of the building.
The most popular day hike is to the pastures at Dajianbao (3750m) which makes for a decent 15km round-trip hike and rewards with majestic mountain views. An excellent overnight hike (ask in your guesthouse for help finding camping gear and a guide) is to the Eldest Sister Peak Protection Station at 4400m. It's right at the foot of the mountains and the imposing glaciers.
At the time of research, a wooden walkway was being constructed along the valley. Once this is complete, visitor numbers are likely to shoot up as the tour bus crowds arrive.