This trip, my kids wanted to see Sumo and it was in Nagoya, so we secured tix and used our expiring JR passes for one last round trip.
Early in the morning, we went to see training at Musashimaru's heya (sumo training area, generally called a "stable" in English). This is early in the morning and absolutely free. It is a wonderful thing to see, and Musashimarui himself was there as well -- indeed, he was kind enough to offer us his car and driver for a ride back to our Nagoya hotel.
Later on, we went to see Sumo in Nagoya and while the "Aichi Prefectural Gymnasium" where it is held isn't nearly as nice as the Tokyo arena, it is much more down-to-earth. In particular, all those old lady cops who, in Tokyo, shoo you away from anyplace interesting were nowhere to be seen. As a result, it is much easier to wander around various portions of the arena. We were again able to have a brief chat with Yokozuna Musashimaru.
The Musashigawa stable trains somewhere in Nakano when in Tokyo. If you are going to be in Tokyo, I STRONGLY recommend getting up early to go see them train. The training is very tough. I am sure JNTO can tell you how to find the various stables to go watch.