As promised, here's my post on the undoo kai:
If you want some reasons that some American kids are so overwieght, I'll give you a couple.
For one, the lunches at Japanese school are very healthy, with lots fish and very few calories.
The kids also spend a large portion of their after school hours at a club, where they play sports from whenever school gets done till about seven(!) at night.
Lastly, the most impressive, they have a festival like day devoted entirely to playing sports. I was very fortunate to be invited by a family of friends to come and watch this, and it was very exciting. Granted, it was a Saturday, so there was no class anyway, but the kids seemed very excited regardless. The classes were divided into two teams, one class on each team from each grade, one to six. The red team and the white team each had their own area to sit. (They brought their desks outside.)
They have this game where there are tires and bamboo poles in the yard. At the start, the teams run and try to gather as many of the poles and tires and deposit them on their side of the grounds. The starter gives another call and the kids then do tug of war with a giant rope. The game is amazing, I saw a group of four kids pulling another from the other team on a tire, because he wouldn't let go.

The cheers that they had were also very impressive. The teams each had a drum and two flags. They did the wave, shouted at each other and shook little rythm cans. The kids who led the cheers had these really long headband that trailed behind them when they ran.
I don't think that this game could be pulled off anywhere but Japan. The game starts with the team divided on each side lined up by fours. One side has a long bamboo(what else) pole. When they start, the do a figure eight around some cones on the ground while running toward the other side. They run straight at the other group, lower the pole and the other side jumps over the pole, in a cascading leap. When they reach the end, the team ducks down and the pole comes back over all their heads. The front four then take the pole and the process then repeats to the other side. I was amazed that the kids were able to do this and words barely convey the wonder of this event.
Lastly, the kids did this arcrobatic dance type thingy where they made pyramids and fans and all sorts of cool balancing acts. They did it in rythm to some cool music and, again, I was very impressed. Speaking of music, there was some interesting music pumping through the loudspeakers. There were some Disney tunes, some cartoon themes and I'm positive that I heard the theme from Back to the Future in there. Well that's it for now.