Rydia's Mathemagic Minute Cheat
In order to get the max score in Rydia's Mathemagic Minute mini-game, quickly memorize the numbers that appear and close the DS. Do the required math, open the DS and input the answers. Repeat this process until you reach the maximum score of 9,999. This mini-game goes to sleep when you close the DS, giving you extra time to do the math. However, you still need to be fairly quick when you input the numbers.
New Game Plus
Once you have completed the game, you are given the option to save. This allows you to start a new game that carries over all of your items and augments from your completed game. This "New Game Plus" will make replaying the game easier, and you should be able to defeat any difficult optional bosses that you couldn't best in your initial play-through.
Whyt's Body Skins
Seven different body skins can be unlocked for the summon monster Whyt. The first five are unlocked by scoring 9,999 points in various characters' mini games. The paladin skin is unlocked by beating Cecil's mini-game. Mastering Kain's will get you the dragoon skin, and the white mage skin is acquired upon completion of Rosa's mini-game. Beating Rydia's game will unlock the summoner/black mage skin, and the ninja skin is unlocked by beating Edge's mini-game. The final two skins are unlocked by meeting different criteria. The goblin skin is unlocked by defeating the game twice (using a New Game Plus) and the Zeromus skin is unlocked by completing the bestiary.
Developer's Room
The developer's room can be accessed from the Dwarven castle in the Underworld. Enter the pub by going through the door between the armor and the weapon shop. Turn right shortly after entering the pub and search the wall until you find a hidden passage. This will lead you to the developer's room, where you can view the game's credits and talk to digitized versions of the game's creators.