If I go to Japan it has to involve Disney. I don't know the specifics but it's repeatedly stated by people its one of the best or THE best Disney park in the world.
The only Disneysea in the world is in Tokyo. Disneysea was meant to be the companion park to Disneyland. It was originally meant to have been built in Long Beach but Disney didn't have the money to do it. So a third party licensed the concept from Disney and built it in Tokyo.
The Disney parks are nice but they would be down the list of things I would do in Japan. Since they are so similar to Disney parks anywhere. I would budget time for things that are unique to Japan. If you do go and don't want to spend all day there, they sell discounted tickets for afternoon admission.
Speaking of which, especially for people in this forum, go to Nakano. Many people have heard of Akihabara which is a must visit. But for popular culture items, there's also this shopping center right next to the train station in Nakano. If you like the small dealers on the exhibit floor at comic-con, this place will be like heaven. If you are into dollar stores, probably the oldest Daiso I have ever been to is at the far end of this shopping center as well. If you are into cameras, particularly used cameras, Fujiya camera is in the very Japanese shopping arcade leading up to Nakano Broadway. Typical Japanese reserve goes out the window when they bring out fresh bins of "junk" camera lenses.
