There are a few secrets in Kingdom Hearts and I will put them here whenever I find them. I will also tell how to view/gain these secrets as well as telling you where they can be found.

Secret Movie Feature

To View the Secret Movie, you must do the following task before defeating the Final Bosses:
– Win the Hades Cup at the Coliseum
– Release all 99 Dalmatians
– Seal all the Keyholes, Including Hundred Acre Woods Keyhole.

Expert Mode Bonuses

After you complete the Expert Mode, your overall Statistics are displayed. In addition to this, a different piece of Art is displayed on ‘The End’ Screen. A 3rd background illustration is Displayed when you completed Expert Mode and gain access to the Secret Movie Feature.