Extra multiplayer content
On the level select screen in multiplayer, tap up seven times to unlock a new stage and multiplayer character.
Infinite Continues
On the continue screen, where it asks Yes or No, use the D-pad to enter the code Up, Up, Down, Down, Left, Right, Left, Right. Each time you enter it correctly, the number of continues remaining will increase. If you're already out of continues you won't be able to do this.
Play as computer
When playing through story mode, have a friend (or two or three) hold the triggers on the other controllers. They must hold it right through the end of each story panel to the beginning of the actual level. They will then be able to control the associated computer player for that stage.