The problem with studying English in Japan is that there are not many ways to study it. Most students either study from a textbook or watch English dramas and movies. Although these are great ways to study, there is little opportunity to actually use the language while living in Japan.
Another way to study English is by taking MOOCs. MOOCs stands for Massive Open Online Courses.
Most MOOCs are similar to any university course you might take. You watch a few video lectures every week, take a quiz, and/or do your homework. Best of all, most MOOCs are free.
Although you can't receive college credit, you can still gain knowledge about areas that you interested in from some of the best universities and institutions around the world.
Currently, I taking two MOOCs, a course on astronomy, and another course about Basic Guitar. Everyday I usually spend about 30 minutes or so, watching the video lectures and afterwards, apply the content by doing the assignments or by taking a quiz.
Although there are great many sites available, I recommend coursera.org/.
Why not learn about something you've always been interested in. You might even learn something new about You!