Get tickets and join the crowds at a game at Nippon Budokan. If there's nothing on while you're there, you might like to check if the stadium offers a tour of the facilities.
This Chiyoda sporting facility is an interesting place to kick off your day of sightseeing. There's loads to do and see in the area, like Tokyo Skytree, Tokyo Tower, and Tokyo Imperial Palace. Take your time at Nippon Budokan before hitting up these other fine sights.