Originally the site of Edo-j?, the largest fortress in the world during its time, the Imperial Palace in Tokyo seems to be all that is left of it. Most of the palace is cordoned off as it is the Emperor's home, but the rest is still open to the public with various guided tours of the castles, galleries, and gardens. The Imperial Palace is just ten minutes from the Tokyo station, making it convenient to reach. The grounds include beautiful cherry blossom viewing spots, well-groomed gardens, and historic ruins. You could either opt for a guided tour of the palace grounds or just go on your own and stop to smell the flowers.
Tours are usually held at 10:00 AM and 1:30 PM from Tuesdays to Saturdays, except public holidays.