It's a hassle when you want to leave your bags and can't find an open coin operated locker. in such cases, use the Coin Locker Navi on which you can search for available lockers. You can search by size (large, medium, small) so it's quite convenient.
If your bag is too big for a locker, go to a baggage storage center (600 JPY per item; there are restrictions on size and weight). They are located in the JR East Japan Travel Service Center on the first floor and on the B1 Floor of GRANSTA inside the ticket gates. You can also have your luggage sent to the airport from these locations.

*The image is for illustrative purposes only.

There is a postbox in the shape of Tokyo Station near the Marunouchi Central Gate (inside the gate). If you post your letter in the slot that's on the left when you're facing it, you will get a special postmark called a "fukei-in" (scenery mark) on the letter. The postmark includes the date and the words "Tokyo Chuo" (in Japanese) and an image of Tokyo Station, so why not send letters to friends and family, and to yourself, to remind you of your trip?

There are many other convenient services and facilities, such as the Station Concierge Tokyo (English speaking) inside GRANSTA. In addition to information about the station and its facilities, the concierge will answer questions about tourism and events in the city, and tourism outside of Tokyo.
If you are traveling with a baby, you'll want to check out the Nursing Room that is equipped with a changing table, sink and nursing chair. Inside the gates, they are located at the Central Transfer Passageway for the Tohoku, Joetsu and Hokuriku Shinkansen on the 1st floor and at the Yaesu Underground Central Gate on the B1 Floor. Outside of the gate, there is one on the B1 Floor of Character Street.
There is also a foreign exchange at the JR East Japan Travel Center on the first floor at the Marunouchi North Exit, as well as in GRANSTA. Both offer exchange for 33 currencies.
*The image is for illustrative purposes only.