Why not walkabout the Imperial Gardens?

Getting there: To Sakuradamon/Nijubashi, take the Yurakucho Line. The Otemon entrance to the East Garden can be reached on foot from JR Tokyo Station or one of the five subway lines (Marunouchi, Tozai, Hanzomon, Mita, Chiyoda) serving Otemachi. The East Gardens are closed on Mondays and Fridays. Hours are 9:00 a.m. to 5:00 p.m until the end of August. Yasukuni Shrine can be reached from JR Ichigaya Station or Kudanshita on the Tozai or Hanzomon Lines. The Imperial Household Agency, East Gardens of Imperial Palace

Read Mark Shreiber’s suggested walkabout at Japan Times who suggests:

“The environs of Tokyo’s Imperial Palace are a great walkabout any time of the year…

My choice for the starting point of the tour was the Hanzomon Gate, about five minutes from Hanzomon Station.
We crossed Uchibori Dori and began descending the hill, walking eastward alongside the inner moat (which is what uchibori means) and its massive earthworks. On the other side of the street we could see the National Theater and the Supreme Court.

At the bottom of the hill, we arrived outside the huge Sakuradamon Gate. Immediately across the street are the headquarters of the Tokyo Metropolitan Police Department and, on the opposite side of Roppongi Dori, the former Ministry of Justice. Designed by two German architects, it was completed in 1895 and currently houses a research institute, library and museum.
Sakuradamon is photogenic, but there’s much, much more to come. Walk through Sakuradamon’s massive old hinged doors and bear to your left, and you’ll find yourself at the vast open plaza in front of two parallel bridges, referred to collectively as Nijubashi (double bridge).
The older bridge in the foreground, the Seimon Ishibashi (stone bridge), has been nicknamed Meganebashi (eyeglass bridge) because its two arches give it the appearance of pair of old-fashioned spectacles; the newer bridge behind it is the Seimon Tetsubashi (iron bridge).
The tower on the right, above the iron bridge, is the Fushimi Yagura. We just fell in step with the tourists, walked right up to the moat and snapped away to our hearts’ content.
The Ishibashi is used as the entrance to the palace on the two public holidays — Dec. 23 (the Emperor’s birthday) and Jan. 2, when the palace is open to the general public. Like climbing Mount Fuji at least once, it’s something every Japanophile ought to experience.
Walking northward through the huge plaza, past the Sakashitamon and Kikyomon gates and the Tatsumi Yagura tower, eventually takes you to the Otemon gate, which is one of three points of entry to the Kokyo Higashi Gyoen — East Gardens of the Imperial Palace.

The gardens — which were part of the palace grounds until they were opened to the public in 1968 — can be entered five days a week, and admittance is free. Upon entering, you will be handed a rectangular plastic chit which, in keeping with the strict security, must be returned to the guards upon exiting, either from the same gate or via either of two gates on the other side of the garden.
Once past the police checkpoint, you’ll pass a rest house on the right that distributes free maps and sells postcards and various souvenirs. Just beyond is a small guard shack (on the right) and larger barracks (on the left) where samurai troops were once posted.
Security, as far as Japan’s rulers were concerned, began at home, with concentric moats plus a series of gates and inner walls, purposely designed with sharp switchbacks to confuse intruders and create bottlenecks that would prevent large numbers of attackers from converging on a single point.
The ramp at Shiomizaka, above a section of the inner moat called Hakuchobori (swan moat), takes you right through the upper walls, and at last you get a detailed perspective of their enormity.
Follow the path along to your right and you’ll see two modern buildings used for classical-music performances, including the Gakubu and Tokagakudo. You then come to a huge open area about the length of two football fields. This was the site of the Honmaru Goten (castle and palace) where the shogun lived and conducted the affairs of state. The buildings were completely destroyed by fire in 1873 and practically nothing remains except the stone foundation of the tenshudai (donjon or fortress) on the north side — the five-story, 51-meter-high tower burned down in the Great Fire of Meireki in 1657 and was never rebuilt.
The short climb to the top of the foundation gives you a panoramic view of the Honmaru area, and the Otemachi and Marunouchi business districts beyond.
After descending, follow the trail for about 80 meters on the right. It leads to a small stone monument and bilingual display panel that mark the Matsu-no-Oroka, one of the most famous spots in Japanese history. This is said to be the actual spot in the palace corridor where, on April 21, 1701, Lord Asano Naganori of Ako Province lost his cool and slashed Lord Kira Yoshinaka, a powerful official, with his dagger.
For this lapse of protocol, Asano was ordered to commit ritual suicide, precipitating in the famous vendetta against Kira on Dec. 14, 1702, by Asano’s 47 loyal retainers (aka the 47 ronin), an act they knew they would pay for with their lives. …” — end of excerpt