Select 'Send' at the main menu, then the 'Download Card Pop-Up Card' option. In the European version, press R, Y, Right, Down, L, A,
Credits mini-game
Tap the letter 'O' when it appears in names and other words during the credits to earn a Silver or Gold rank and a card.
Get Omega Cannon card
To get the Omega Cannon, select "Send" from the main menu and "Download Card." Press the L button twice, then Up, the B button twice, then Left on the D-pad.
Kingdom Hearts references
Get to the house. Look at Croma and note the strong resemblence to Vivi from Kingdom Hearts. Go to the Mog's shop. Stand in the circle and note that the symbol is actually the Kingdom Hearts symbol.
Lightning tower password
Enter "62 20 39" as a password.
Music Player
In order to unlock the music player, complete the Story Mode. This option will then be available in the Sound menu, available in the main screen, by clicking the purple option in the lower part of the screen.
Omega Cannon
Select "Send" from the main menu and "Download Card." Press the L button twice, then Up, the B button twice, then Left on the D-pad.
Omega Card
Go to "Download Pop-Up Card" and enter the code while it says "Looking for available Downloads".
Right, L, Y, Right, Down, Up.
Promo cards
Use a Nintendo DS Download Station to get the promotional cards Omega Cannon, Ultima Weapon, Ragnarok, Shinryu, and Chocobo.
Ragnarok card
Select 'Send' at the main menu, then the 'Download Card Pop-Up Card' option. In the European version, press A, L, Left, Up, R(2).
Shinryu card
Select 'Send' at the main menu, then the 'Download Card Pop-Up Card' option. In the European version, press Down, X, R, Right, A, B.
Ultima Weapon card
Select 'Send' at the main menu, then the 'Download Card Pop-Up Card' option. In the European version, press L, Up, Right, B, A, B.