Hold A + B + X + Y + Start during game play.
Challenge mode option:
Save the game at any point to unlock a "Challenge Mode" option
at the main menu. Select that option to replay previously
completed sections in a time attack type scenario. Collect as
many Pikmin as possible before nightfall.
Press Down to round up four Pikmin to carry you back to your base camp.
A short fireworks display appear. This also eventually happens if
you stand idle.
Invincible Captain Olimar:
Press Down and Captain Olimar will lay down.
He cannot be harmed in this position.
Remain idle at the level selection menu for about five to ten minutes.
You will hear a different song playing in the background.
This also occurs in Challenge Mode.