Check out Ryogoku
If you go to Ryogoku, you can find a wide variety of sumo stables and see sumo wrestlers going about their daily business. Ryugoku has its own sumo stadium as well, called the Ryugoku Kokugikan. You can watch sumo wrestling matches there on occaision. I also recommend trying Chanko Nabe, which is a heavy Japanese stew that sumo wrestlers regularly eat in order to gain weight.