You can take bus No.970 out of Dongzhimen Gate to Miyun Bus Station. The bus runs from 05:40 to 19:00. You should start early and take the first bus. When you arrive in Miyun Bus Station, hire a car to Simatai Great Wall. Spend less than four hours on the wall and return. On the way back, you can hire or share a car at Simatai Parking lot, or take any bus which goes to Beijing at the entrance of Simatai Great Wall. Take your time.