You have to book to get into a lot of the grounds, but there is one section of the park that is free and open to the public during set hours. It is stunning in Autumn with the ginkgo trees and autumn leaves.
Be aware that you likely won't get to enter some sections of the park, depending on what time of the year it is. I was told that the whole area is open during the New Year and the emperor's birthday. Not sure about that though.
On a sunny day, it's fun to visit the palace and enjoy the sunlight. There is an open field in the middle of all of the greenery; it's barren but somehow goes along with the whole setting. You can also explore a small museum in the palace that displays multiple traditional Japanese ink prints. What's a real adventure is walking around the whole perimeter of the palace. While walking around, you can see a blend of the traditional architecture of the imperial palace and the modern architecture of the rest of the city in the background.
I think it's only open during specific time periods. There should be more information on their official website. Nevertheless, I think it's still worth visiting and walking around. It's near the Tokyo station and has many department stores nearby as well.
I would recommend taking some time to visit the gardens if you're going to the palace anyway! It's a peaceful, tranquil spot.
Beautiful park to stroll through in any season.
Tip: On 23rd December, it's the Emperor's birthday, and it's the only day next to New Year when you get to see the Emperor and enter the palace grounds.
This place is so green during summertime. I recommend bringing umbrella or sunblock if you're planning to travel on summer. The place is huge and the surroundings are amazing. Tourists are not allowed to visit inside palace grounds. There is only a specific month and day where you're allowed to enter inside. I recommend to check out jgl tours in Japan if you want to visit this place.
The is an available free completely guided walking tour inside the Imperial Palace grounds (but not inside the buildings). You need to line up outside the Kikyomon Gate and wait for the passes to be distributed. Distribution starts at 8:30 am - noon. Only 300 visitors are allowed per session. Just bring your passport for identification.
The tour is in Japanese. You will only have one tour guide but will be surrounded by security. There is a separate tour where they will provide you an audio recording of all the things discussed by the tour guide.
While you're at it, make sure to buy at least one of the sweets from the small souvenir shop inside the Palace. It's called Kunaicho Seikatsu Kyodokumiai (Imperial Household Agency Cooperative). Most of their products (with the Imperial Palace insignia) are only available there. Also, I can attest that their sweets are delicious. They are great with tea.
Throughout most of the year, you are only allowed to walk around the Imperial Palace gardens. With that being said, you can spend up to a couple of hours there, as it's huge! If you're pale skinned like myself, then I would recommend bringing sunblock with you so that you don't burn. There are lots of open areas where it's difficult to walk in the shade on a very hot and sunny day. You have been warned!
As I walked through the gardens, I noticed several tours taking place. If that's something you would like to have while you're in the gardens, check online before you go to see if you can reserve before you get there. The palace gardens are a nice escape from the busy parts of Tokyo and can help you relax . There are several rest areas and shelters where you can get away from the sun. They all should have vending machines, too. There are plenty of water fountains scattered around the gardens, as well.