Everyone has said the best stuff but I'll add something. Go buy some porn. It's such an odd experience. Everyone is totally cool and chill about being surrounded by some really odd porn. I felt strange for about 2 minutes then I was flicking through magazines like the best of them. It's more for the 'wow this is something ive never done' more than the WOO PORN factor.
Also go to an Onsen (spa/sauna) I went to an outdoor one in Tokyo and it might have been the most relaxed I've been in my life.