Secret Battle Stages

Go to the game selection screen (where you choose Adventure Mode, Battle Mode, etc) and press Start repeatedly as quickly as possible until you hear a chime. Now, in Battle Mode you should have four extra stages.

Secret World

To gain access to the new world, collect all 100 Gold Cards and beat Altair again.

Sound Test

Beat the game in Adventure Mode and you should be able to access the sound test option.

View Credits

Beat the game with 100 Gold Cards. Then, beat the extra world by defeating Sirius. You should be able to select viewing the credits from the options menu.

Hidden Arenas

Go to the Title Screen. Highlight "Battle" and press Start quickly until you hear a tone. Four extra arenas will then be available.