If you want to meet new people and explore your interests while you are in Japan, one good way is to take classes. There have always been a scattering of "open classes" 公開講座 at community centers, but now that Japan has such a large aging population there is a huge choice of topics for you to enjoy.
Think you are too young to mix with senior citizens? Possibly, but keep in mind that these are people with enough spirit and energy to get out and take classes. They may have enough time and interest to talk with you and this will open up new doors for you. When I was studying in Japan I took a course on Japanese women's history, another one on storytelling, and also attended talks in public libraries. It got me out of my dorm, away from the other foreign students, and provided an opportunity to explore my interests and make new friends.
Every town and district in cities should have a kominkan. You should find flyers for events at your local public library or grocery store. Check it out, you won't regret it.