Go on a clear day if you have a choice. The northwest of the city is a metro ride plus a short walk or a car. It does not combine neatly with the Forbidden City except for the very energetic.
Beijing · 颐和园
Summer Palace
Kunming Lake is the point. The painted walkway and the hill of the Tower of Buddhist Incense are the set pieces. It is a park with palaces in it, not another Forbidden City.