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There are two main rewards available for travel within Asia using CO miles, the Series 8 and the Series 8B. The main difference between the two are the airlines permitted. Right now you are pretty limited in options for travel intra-Japan (I actually don't think there are any options unless you connect flying on KE) but around November CO will be joining StarAlliance, bringing ANA on as a partner. Of course, when that happens there is a very strong likelihood that the reward charts will be rewritten so the number of miles required will change.

My guess is that intra-Japan travel isn't going to be the best use of your points; look for longer, more expensive trips to get a better value.