To get secret costumes, you'll have to smash something in a certain level. To get that character's costume though, you need to smash the level with that character.

Tom the Cat
Cowboy Kitty, found in Unfurgiven
Muskateer, found in Luncheons and Dragons
Beach-Trunks, found in PAWS
Sailor Scout, found in That Sinking Feline

Jerry the Mouse
Mousekateer, found in Luncheons and Dragons
Snow Parka, found in Snow Fight
Beach-Trunks, found in PAWS
Sailor Scout, found in That Skinking Feline

Spike the BullDog
DevilDog, found in Scrapyard Scrape
SherriffSpike, found in Unfurgiven

Butch the Cat
Beach-Trunks, found in PAWS
Auto Scrubs, found in Scrapyard Scrape

Nibbles the Mouse
Mousekateer, found in Luncheons and Dragons
Indian-Chief, found in Unfurgiven

Duckling
Snow Parka, found in Snow Fight
Coon'Skin Cap, found in Unfurgiven

Unlock new playable cast characters by clearing challenge mode with existing and unlocked characters.