Finish the game as any character to unlock a bonus mission at night for that person.
Fight against Gaara
Finish the game as any character, then play again as that same person to fight Gaara as the end Boss.
Gallery
Finish the game as any character to unlock a gallery of pictures at the main menu.
If you want to unlock every gallery picture, use all three characters to Finish the night missions with every one of the 100 Leaf icons.
Sound test
Finish the game with all 100 Leaf icons to unlock the "Sound Test" option at the main menu.
Bonus level...