Osaka-joh (大阪城), the Osaka Castle is really worth seeing, even though it is in fact only a concrete reproduction of the original building (japanese people are not really sensitive to the original character of their buildings; the Kinkakuji is by the way also a reproduction).
The inside of the castle has unfortunately not be reconstituted at all. Instead, you will however see a museum, which outlines the history of the castle and of Taikoh Toyotomi Hideyoshi, considered as the unifier of Japan.