The Imperial Palace, in the very heart of Tokyo, is not only a major green area but the seat of Japan’s Imperial House since the Meiji Restoration in 1868, when the capital moved from Kyoto to Tokyo. The whole area is very large and from outside you can just glimpse at some of the buildings inside the walled fortress. It feels very medieval in a way, with the moat, the huge stone walls and the fortresses. You can have a more panoramic view from some of the hotels in the area, most notably the Peninsula (go up for a glass of Champagne at the bar).
If you want to visit the interior of the area, you need to book in advance. There are guided tours lasting around 75 minutes twice a day, at 10am and at 1:30pm. The whole application process is very well explained on the website. If you are also visiting Kyoto, go ahead and book your visit to the Palace over there as well.