Funabashi H.C.Andersen Park is a theme park that is compared to Disney Land and Disney Sea. Sure, you can't get many rides there (except pony rides), but there are plenty of things to do there for a family. It is easy to spend a whole day there without even noticing. The park is named after the famous fairytale writer, Hans Christian Andersen. He was born in Odense (Denmark), the sister city of Funabashi.
The park has 5 zones: Kids Kingdom, Fairy Tale Hill, Children's Museum, Nature Experience, and Castle of Flowers. Perhaps the most interesting zone for adults is the Fairy Tale Hill Zone. You can ride a boat in the Pond of the Sun, visit the Community Center and buy some souvenirs, see Andersen's Statue, and look around the Farmer's House. If you like walking in nature, take a relaxing stroll in the Nature Experience Zone.
For kids, a good option is to go to the petting zoo, the Children's Museum, or the Kids Kingdom Zone.