Professor Layton and the Diabolical Box [NDS]

Unlockable: The Animal Lover's House (Layton's Challenges)
To unlock the Animal Lover's House in Layton's Challenges mode, clear Level 1 on the Hamster minigame.

Unlockable: Final Puzzles for Layton's Challenges
The Bonus Menu features an option called Layton's Challenges. The extremely tough puzzles can be unlocked by playing the game. When you beat the game, the final puzzles -- 148, 149, 150 -- are unlocked ("The Sweetheart's House").

Unlockable: Character Profiles
Beat the game to unlock all of the Character Profiles. These can be found in the Top Secret section of the Bonus Menu.

Unlockable: Art Gallery
Beat the game to unlock the Art Gallery. It can be found in the Top Secret section of the Bonus Menu.

Unlimited Hint Coins
Each time you need to use a hint coin, fret not! Your supply will never diminish if you just save before each puzzle and reset the game after viewing all of the hint coins.