The sumo tournaments are totally amazing and if you have a chance to go you should definitely go. There isn't always a major tournament on though so the sumo training sessions are an amazing experience toease the pain of missing the big tournaments.
One of the great things about the training sessions is that you get way closer to the sumo. It doesn't have the ceremony of the tournaments but you feel like you are a part of the session.
Sumo are revered in Japan and for the people who follow along it is a serious business so it is a rare and special experience seeing the training sessions.
For more information head to Sumo Training