1. Study for the work you will be experiencing.
2. Study Japanese.
If you're wondering what to do now, with all the home country amenities afforded to you, what are you going to do between classes here? If you're like a JET ALT, you waste so much time surfing the Net, posting inane stuff on BigDaikon.com, and complaining about it.
Not any more. Things changed about 3-5 years ago.