We visited Odaiba on a Friday afternoon; it was sunny, but chilly, so there weren't a lot of people out on the beach (except a lot of workers setting up for a professional bike race to be held there). It is a nice and pleasant walk along the water. You can get good views of the Rainbow Bridge and the main part of the City. There is some kitsch, like the huge ferris wheel and the mini Statue of Liberty. Lots of "beach" themed shopping and restaurants. There is some interesting architecture in the area: take the Yurikamome train from end to end and you will see a lot of interesting buildings. There is also a sightseeing boat you can take; it departs regularly from a prominent pier off the boardwalk.
There are a number of things to see, including a maritime museum and a number of arcades/entertainment centers. We didn't have a lot of time, but we still liked getting out there and seeing the area.
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