Complete the following levels and the tasks in them to unlock the corresponding hats.

Unlock Crown of Flowers:
Beat Level 1-1 with gore off.

Unlock Stove Pipe Hat:
Beat Level 1-1 with gore off.

Unlock Pirate Hat
In Level 1-1 destroy 'Hairy Mommy Daycare'.

Unlock Blond Wig/Pearls:
Beat Level 1-2. Unlock Conical Purple Hat:
In Level 1-3 destroy 'Castle of Crap' building.

Unlock Brown Wig/Pearls:
Beat Level 1-3.

Unlock Chef Hat:
Beat Level 1-3.

Unlock Red Bandana:
Beat Level 1-4.

Unlock Jheri Curl:
In Level 1-4 destroy 'Fish are like Plants' building.

Unlock Rooster Hat:
In Level 1-4 destroy the Behemoth-logo building (must be out of tank).

Unlock Black Afro:
Beat Level 1-5.

Unlock KGB Agent Hat:
Beat Level 2-1.

Unlock Tiara:
Beat Level 2-2 on Hard difficulty setting.

Unlock Daisy-Petal:
Beat Level 2-3.

Unlock Yellow Shark Fin:
In Level 2-4 dig/suffocate before riding snowmobile

Unlock Brown Hat:
Kill 7 birds in Level 2-5. Unlock Private Eye Hat:
Beat Level 3-2 on Hard difficulty setting.

Unlock Area 51 Agent Hat:
Beat Level 3-2.

Unlock Jester's Cap:
Beat Level 3-4 on Hard difficulty setting.

Unlock Nurse Cap:
Get to the final Boss in Level 3-5 (you don't have to win).

Unlock Brown Hair:
On Hard difficulty setting when you have beaten the final boss jump off the spaceship before the screen fades.