When you get the final sword in Fujimi Shrine, you get 15,808 experience. Throw the sword away and talk to the holy beast again. You can repeat this as many times as you like.

After you beat the game, at the final screen that says "the end" Ко, press run to see some extras.

Plug in a 3rd controller and press RUN at the title screen. Ziria will say "yatta!" and you'll go to a special screen.
Press a button on the 1st controller to see the story.