The Imperial City may actually have been more important to Chinese history than the Forbidden City it surrounds. It was where the government and those that made the laws of the land lived and worked. It's scale was dramatic; from Houhai lake in the north to Qianmen in the south, it encompassed Behai & Zhongshan Park in the west, and the remains of its Ming Dynasty East Gate can be found near Wangfujing Street. Yet the scale of the imperial museum is quite the opposite. The museum is quite small and has little on display. It has a great diorama showing the scope of the imperial city as it was. But the same kind of piece is viewable in the Beijing Urban Planning Museum with many other items to also hold your interest. This is not the museums fault as almost all of the remnants of the Imperial City were destroyed over the last hundred years in the name of progress. Still, it is one of the few museums in the city that might not be worth bothering with, which is a real shame considering the importance of the area that it represents.
It does have a pleasant park, Changpu River Park, that follows a piece of the old imperial wall just to the east of Tiananmen Square. It is a great place to catch your breath after dealing with the hordes of tourists in Tiananmen and the Forbidden City. Best of all the park is free! Both the museum and the park can be found by facing Mao's Portrait and turning right. Follow the wall and make your first left onto a small hutong.
The park is right there. Beyond the park, are many stroll worthy hutongs between the Forbidden City and Wangfujing Street. If you want to go from one to the other, stroll through Changpu Park and then down through the park next to Huangchenggen Jie. It will keep you in green surroundings for most of the trip. You will have traced part of the old imperial wall and seen much more than can be found in the museum itself.