The Nikkan Festival is coming to Tokyo's Komazawa Park in late September. An exchange of cultures between Japan and Korea, the festival represents a celebration and coming together of both nations.
First held in 2009, the festival is staged in both Tokyo and Seoul over two days. The festival features numerous stage acts including performances by popular K-pop artists. In previous years, visitors have witnessed taekwondo martial art displays, cultural dances and singing performances, Korean acrobats, Japanese traditional arts as well as foods from both countries.
The event is a free one and for those interested, volunteering opportunities are available.
As for sumo, it's a great experience. Seriously! Check out this guide https://en.japantravel.com/tokyo/guide-to-sumo-at-the-kokugikan/59643#where-to-buy-sumo-tickets
Hope that helps!