A day trip to Jiankou is definitely possible. You could take a taxi or bus to Xizhazi in the morning, climb up to
Zhengbeilou and from there to
Mutianyu via
Niujiaobian. From Mutianyu, you can easily find transportation back to Beijing.
But if you stay overnight, you can visit other parts of Jiankou, like
Jiuyanlou, which offers amazing views.