Big Head Mode
Hold L + R at the title screen, and choose your character with those buttons held. The character will now have a gigantic head.

Different Costume Colors
Hold Down and press A, B, or C when selecting a character to alternate between the various colors available to them.

Fight as Bosses
Press Up, Down, X, B, A, Y, C, Z, Start at the title screen. Now you will be able to play the boss characters by simply selecting them at the character select screen.