Let's go to the sumo tournament at Ryogoku stadium in Tokyo. We have been three or four times before, and it is great. To get cheap tickets, we will have to go early. You can read our guide to sumo at my blog at: http://richandjoan.blogspot.jp/2012/09/information-on-tokyo-sumo-tournament.html
We will go early to buy tickets and maybe go for an urban hike before the senior division starts in the afternoon. Or you could go to the Tokyo Edo museum next door - one of the best museums in Japan.