For stupefying hikes along perhaps Beijing’s most incomparable section of Wall, head to Jiankou, where white-knuckle sections like ‘Upward Flying Eagle’ and ‘Heavenly Ladder’ will make you marvel at just how they built the thing in the first place. Accessible from Xizhazi Village (西栅子村, Xīzhàzi Cūn) via the town of Huairou, and connected to Mutianyu Great Wall, you'll need a car and driver to hike here, and someone to guide you up onto the wall.
Be aware, too, of the dangers. The battlements are unstable, weather is changeable and you're a long way from help should you run into difficulties.
You can access the Wall from a number of points along the valley. Most visitors hike up along woodland paths to Zhengbei Tower (正北楼, Zhèngběi Lóu), the best lookout in the area. It's too steep and dangerous to hike west along the Wall from here, but the views are mind-blowing. Going the other way, you'll get to Mutianyu Great Wall in about 2½ hours (mostly downhill), passing the Ox Horn (牛角边, Niú Jiǎo Biān; 90-minute walk to Mutianyu), which performs a sweeping, 180-degree U-turn on the hillside.
Another popular hike is to get on the wall further west, skipping the more dangerous sections like Upward Flying Eagle (鹰飞倒仰, Ying Fei Dao Yang). Hikers mount the wall near a section called the Beijing Knot (北京结, Běijīng Jié) where the Wall splits in two directions. From here you can hike northwards (with some hairy bits to circumvent along the way) to the Nine-Eyed Tower (九眼楼, Jiǔ Yǎn Lóu, 2½ hours), a lofty lookout so named for the nine windows in the watchtower, which offer views of the full sweep of Jiankou on a clear day.
Xizhazi Village has several farmer-style courtyards (农家院, nóngjiāyuàn) at which to spend the night and dine. Most of these are authentically rustic with heated 'kang' beds, musty blankets and framed portraits of Mao on the wall.
From Dongzhimen Transport Hub, you can take bus 916快 (快 is ‘kuài’, or ‘fast’) to Huairou (¥13, 1½ hours, 6.30am to 7.30pm). Get off before the terminus at Mingzhu Guangchang (明珠广场) bus stop, where you'll find drivers who will take you to Xizhazi Village for about ¥150 one way (one hour).