Cheat Gnome
Quickly press B, Z, Up, Down, B while on a lot to spawn the Cheat Gnome. It appears at different locations, depending on which island you are on, and must be found. The following codes can be enabled, which will unlock the corresponding options on the Cheat Gnome after it is found.
Add one to skill
Quickly press 2, Up, Right, Z, Right.
All crafting resources
Quickly press A, A, Down, Down, A.
All motives
Quickly press Minus, Plus, Z, Z, A.
All plans
Quickly press Plus, Plus, Minus, Minus, Z.
Bonus outfits
Pause game play, then press Z, Z, A, Z, 2 at the "Fashions And Grooming" screen to unlock a vest for a male and a tank top for a female.
Maximum current food and resources
Quickly press Left, Right, Left, right, A.
Maximum relationships
Quickly press Z, Plus, A, B, 2.