Fastest piss on land
165mph on Shinkansen from Nagoya to Kyoto
Kobe beef - equal to and maybe even better than Argentinian beef
Natto - I tried it twice, it's just wrong
Most consecutive days eating rice
All of them
World's hardest game
Spot the fat person. Seriously, sumo wrestlers aside, after 2 weeks, I'm winning 2-1, and I suspect one of mine was American.
Highest slipper quotient
It's currently running at 3.8 pairs per hotel/guesthouse. The Japanese love their slippers. There's a different pair for every occasion.
Pending the results of further travels Japan is also a very strong contender for friendliest people, cleanest country and is almost a shoe-in for best baths (*sigh* I'm going to miss my daily onsen)
Finally, a side-award for best book to read when when visiting Japan - so sorry, but it's got to be Shogun (thanks for the tip-off Sheena)