What would be the best way to volunteer in Japan?

I am a Japanese-American that can speak fluent Japanese. I am currently in Saitama-Ken helping out family friends with their own business. I also have plans to head south to Kochi-Ken to meet some family. Is there anyway I can volunteer if my work schedule ends July 15, even for a couple days then hop on a bullet train back to Kochi? Once I get to Kochi I plan on spending a few days before I hop on a flight back to Haneda airport, and depart Japan on July 24 back to America.