be careful of sushi, buy some slippers,
do NOT blow your nose inside a restaurant,
learn how to say rest room in japanese,
Be polite,most Japanese speak english,enough to understand you.

Get your "shots" ask you DR> what they recommend,Like hepatitis,etc
Bring extra Prescribed meds,on the plane carryon bag,along with a Current note from your DR. stating what and Why you take these meds..
check to see if you can add a health and life insurance rider to cover you in Japan
Bring extra $$ small bills for tips etc..
Bring extra toilet paper,toothbrush,toothpaste shaving stuf etc as needed
if wear glasses,extra pair,even old ones,
Copy of your passport/proof of citizenship,mail copies of these to the place you have a room reserved to stay...
Do you belong to AAA,or AARP,etc, ask them for more hints...