Complete all eight Dreams with a "C" or better rank.
At the title screen, press Left, Right, X, B, A, Y, RB, RT, Down, Up, LB, and LT in that order. If done correctly, you'll get a confirmation that everything in the game has been unlocked, though save data, achievements, and posting to leaderboards will be disabled until you restart the game from the Xbox Dashboard.
Complete Christmas NiGHTS with Elliot and Claris to unlock the "Nightopian Collection" at the main menu. This option displays the Nightopians, Mepians, Nightopian and Mepian eggs in each level; and the mood of the Nightopians and Mepians in that level.
To unlock Christmas NiGHTS, simply complete all 8 dreams with a "C" rank or better.
Display The Music Version Played In A Level
In each level, music changes according to the mood of that level's Nightopians (which you can check with the "Nightopian Collection" option from main menu). In order to show the music version played in the current level, while in "pause", press L1 + R1 + triangle simultaneously.
At the title screen, press Left, Right, Square, Circle, X, Triangle, R1, R2, Down, Up, L1, then L2. If done correctly, a message will appear telling you everything's unlocked, but you can't acquire trophies or upload your highscore.
If you complete Christmas NiGHTS as both Elliot and Claris, you will unlock the option to view the Nightopian Collection on the main menu. This will show the Nightopians, Mepians, Nightopian/Mepian eggs in each level, as well as the mood of the Nightopians and Mepians in that level (the music alters based on how happy or sad they are).