[Nyororin]

I can`t really offer any suggestions for software...
But if you have a little bit of cash to invest, it may be easier to just buy a portable navi in Japan. You can pick up last year`s models used for less than 10000yen.

Of course, it won`t be in English and will be something else to carry around.
Otherwise the best bet is probably to rent a phone and use it`s built in navi system. Mine pulls from either Google Maps or Zenrin - both no extra charge to use. Just the unlimited packet plan to pay each month.