While the film was being made, Jonah Bobo (Danny) lost four teeth, so the filmmakers made 'flippers' or fake teeth that he wore, since the movie takes place in only a day. (imdb.com)
In every Chris Van Allsburg book, there is a bull terrier named Fritz. Jon Favreau put Fritz in the movie - look for a Beanie Baby bull terrier behind Walter near the end of the movie. (imdb.com)
Originally conceived as a direct sequel to Jumanji (1995). The visual effects supervisor on that film, Ken Ralston, was lined up to direct before Columbia abandoned the project in favor of adapting Zathura. (imdb.com)