New characters:
In 1P mode, go to the start circle on the main map. Press Left, Left, Up, Down, then simultaneously press L1, L2, R1, and R2. A character select screen appears. Who you wanna be?
Special Stages:
At title screen with the flashing "Press Start Button," press R1, Up, L2, Down. A little green dude will show up in the bottom right. Select a Puzzle Game and you can play on new backgrounds.
More Credits:
At Title screen, pick "Options." Now press Left, Right, R1, R2, L2, L1, Up, Down. A timer should appear, counting down from 30. Start hitting X like crazy....the more you do, the more credits you shall receive.