Take bus 980快 from Dōngzhímén Transport Hub (Dōngzhímén Shūniǔzhàn) to its terminus at Mìyún bus station (密云汽车站; Mìyún qìchēzhàn; ¥17, 100 minutes, 6am to 8pm). The 快 (kuài) means 'fast'. Then, turn right out of the bus station, cross the main road and turn right and walk for 200m to find the stop for bus 25, which runs to Gǔběikǒu (¥10, 70 minutes). To catch the bus back to Mìyún from Gǔběikǒu, walk through the tunnel by the entrance to the village, cross the road and the bus stop is 400m ahead of you. The last bus 25 back to Mìyún leaves at 4.35pm. The last bus 980 back to Dōngzhímén is at 6.30pm.
Taxis cost ¥1000 to ¥1200 return for a day trip from Běijīng.