Here's a list of all the prizes I have heard about so far. It doesn't seem to matter which world you complete, the prizes are given out randomly.

Paint Brushes
Darigan Paint Brush
Pirate Paint Brush
Maraquan Paint Brush
Faerie Paint Brush
Mystery Island Paint Brush
Lost Desert Paint Brush
Pink Paint Brush
Ghost Paint Brush
Fire Paint Brush
Usuki Paint Brush
Spotted Paint Brush
Cloud Paint Brush
Starry Paint Brush
Disco Paint Brush
Rainbow Paint Brush
Electric Paint Brush
Glowing Paint Brush
Orange Paint Brush
Silver Paint Brush
Split Paint Brush
Gold Paint Brush
White Paint Brush
Shadow Paint Brush
Scritchy Sketchy Paint Brush
Strawberry Paint Brush
Skunk Paint Brush
Checkered Paint Brush
Invisible Paint Brush
Christmas Paint Brush
Brown Paint Brush
Yellow Paint Brush

Petpet Paint Brushes
Faerie Petpet Paint Brush
Plushie Petpet Paint Brush
Pink Petpet Paint Brush
Grey Petpet Paint Brush
Black Petpet Paint Brush
Robot Petpet Paint Brush
Tyrannian Petpet Paint Brush
Halloween Petpet Paint Brush
Ghost Petpet Paint Brush
Purple Petpet Paint Brush
Brown Petpet Paint Brush
Orange Petpet Paint Brush
Blue Petpet Paint Brush
Green Petpet Paint Brush
Spotted Petpet Paint Brush
Glowing Petpet Paint Brush
Christmas Petpet Paint Brush

Miscellaneous
500 Dubloon Coin
10 Dubloon Coin
Cool Negg
Kauvaras Potion

Codestones
Eo Codestone

Bri Codestone

Zei codestone

Har Codestone

Vo Codestone

Orn Codestone

Petpets
Snowickle

Meepit
Gallion
Khonsu
Mortog
Gwortz
Feloreena
Acko
Ona
Mirgle
Screal
Faellie
Whinny
Reptillior
Apis
Trumadon
Taweret
Dragoyle
GX-4 Oscillabot
Pawkeet
Symol
Primella
Bearog