Datong to Changping
Yungangshiku (Yun Gang Grottoes) is a nice site, as is Xuankongsi (Hanging Monastery), near Datong. G109 is hilly as all get out but the decent back into Beijing makes up for it. Do it in 2 days for a challenge, 2.5 days and enjoy the ride a bit. If you get to Guangling after seeing the two sites by the end of the first day then making it back to Beijing is feasible. Also consider staying on S342 up to the G110 for a flatter route back into norther Beijing.