Hi danholo,
If your main focus is learning Japanese then a language school might be best. I think you can work up to 20 hours each week while studying (on a student visa)--but first you`ll need permission from the immigration authorities to engage in an activity that falls outside the purview of your original visa. And to obtain this permission you`ll need to find an employer to prepare the proper paperwork on your behalf.
You could also try working on a farm in rural Japan--try the WWOOF website, which has lots of listings. I don`t think anyone can/will sponsor a working visa if you`re coming to do farm work, but since you probably won`t be earning any income (just room and board, as far as I can tell), then a nintey-day tourist visa would seem to suffice. Caveat: I`ve never participated in WWOOF, so I can`t vouch for it. Have often thought of giving it a try, though...
Good luck!