At the main menu, highlight Options and then enter any of the following cheats:
2D Characters: simultaneously press and hold R1 and L1, and then press O, O, O, O
Christmas Apu: simultaneously press and hold R1 and L1, and then press triangle, triangle, O, triangle
Halloween Bart: simultaneously press and hold R1 and L1, and then press triangle, triangle, O, X
More Camera Views: simultaneously press and hold R1 and L1, and then press triangle, triangle, triangle, triangle
More Money: simultaneously press and hold R1 and L1, and then press square, square, square, square
New Year's Krusty: simultaneously press and hold R1 and L1, and then press triangle, triangle, O, square
Night Time Mode: simultaneously press and hold R1 and L1, and then press X, X, X, X
Nuclear Bus: simultaneously press and hold R1 and L1, and then press triangle, triangle, square, X
Smithers: simultaneously press and hold R1 and L1, and then press triangle, triangle, square, square
Soapbox Car: simultaneously press and hold R1 and L1, and then press triangle, triangle, square, O
Thanksgiving Marge: simultaneously press and hold R1 and L1, and then press triangle, triangle, O, O
View Collision Lines: simultaneously press and hold R1 and L1, and then press triangle, triangle, X, X