Ok here's how you do:

1.Get a Silver or Gold Trophy (It's more easier to get the Gold Trophy.how to unlock Gold trophy ,Unlock all stories from challenge mode)
2.At the Main Menu Screen, press the following buttons to activate SandBox Mode: up, up, down, down, left, right, left, right, B, A and you can now activate SandBox Mode.

Codes for SandBox Mode:

In SandBox Mode, here are some buttons to summon Items, Fishes and even Pets (Including BONUS pets!):
q - stinky
w - niko
e - itchy
r - prega
t - zorf
y - clyde
u - vert
i - rufus
o - meryl
p - wadsworth
a - seymour
s - sharpel
d - gumbo
f - blip
g - rhubarb
h - nimbus
j - amp
k - gash
l - angie
; - presto
z - m= aliens

(Note: Make sure the "Caps Lock" is "off")

BONUS PETS
A=brinkly
S=nostradamus
D=walter
F=stanly

(Note: Make sure the "Caps Lock" is "on")

The Cheat Code (from TWF (Thumb Wrestling Federation): If you know this series, you also seen an episode that Wasabi vs. Corbata was fighting. at that episode, sometime he will use his Signature Move: The Cheat Code (the Cheat Code is The SandBox Mode code). I'm done.