5th Puzzle Mode:

At the main menu, select the second option, then hold A and B on Player 2's controller. During this time, press A on Player 1's controller to unlock a fifth puzzle mode.

 

Access levels 7-12 of challenge:

When you select "start challenge" (3rd from top) on the screen, push A and B on controller 2, then A or start on controller 1. You can now choose from levels 7-12 instead of 1-6.