Secrets SSX is packed full of cool codes and secret things to unlock. From the following page from IGN Codes , you will find details on all of the various tips and tricks in the game.
Boards Unlock all of the other boards by achieving new levels of rank.
Costumes Unlock the other costumes by performing all of the various colored tricks. Third costume Complete all green circle tricks to unlock the third costume. Fourth costume Complete all blue square tricks to unlock the fourth costume. This code has to be entered from the Options Menu. There should be a tone to confirm that the code worked.