Marunouchi is the district between Tokyo Station and the Imperial Palace — the business heart of the city by day, and one of the best areas for a walk. The Imperial Palace grounds are vast: from the Tayasumon Gate in the north to Hibiya Park in the south is a serious stroll.
Within the great moats, one thing is conspicuously missing: a grand imperial palace. The Emperor's residence is there but it is modest and largely hidden from view. The real attractions are the massive stone walls and gates, the East Gardens (free to enter and beautifully maintained), and the several museums scattered around the grounds. You could easily spend a day visiting all of them. This is also one of the greenest parts of central Tokyo — the lungs of the city.
The Marunouchi side of Tokyo Station has been restored to its original 1914 red-brick facade and is worth a look in its own right. The surrounding streets have upscale shopping and restaurants.