Unlock the following hats by performing the corresponding tasks:

HAT - HOW TO UNLOCK
Area 51 Agent - Beat level 3-2.
Black Afro - Beat level 1-5.
Blond Wig/Pearls - Beat level 1-2.
Brown Hair - Defeat the final Boss on the Hard, then jump off the spaceship before the screen fades.
Brown - Kill seven birds in level 2-5, then Beat the level.
Brown Wig/Pearls - Beat level 1-3.
Chef - Beat level 1-3.
Conical Purple - Destroy the "Castle Of Crap" building in level 1-3, then Beat the level.
Crown Of Flowers - Beat level 1-1 with gore off.
Daisy-petal - Beat level 2-3.
Jester's Cap - Beat level 3-4 on the Hard.
Jheri Curl - Destroy the "Fish Are Like Plants" building in level 1-4, then Beat the level.
KGB Agent - Beat level 2-1.
Nurse Cap - Reach the final Boss in level 3-5.
Pirate - Destroy the "Hairy Mommy Daycare" building in level 1-1, then Beat the level.
Private Eye - Beat level 3-2 on the Hard.
Red Bandana - Beat level 1-4.
Rooster - Destroy the Behemoth logo building while out of the tank in level 1-4, then Beat the level.
Stove Pipe - Beat level 1-1 with gore off.
Tiara - Beat level 2-2 on the Hard.
Yellow Shark Fin - Dig or suffocate before riding the snowmobile in level 2-4, then Beat the level.

ACHIEVEMENT - HOW TO UNLOCK
Alien in Training (5 points) - Complete level 1-1.
Hungry Yeti (10 points) - Eat 50 KGB Agents with the Yeti.
City Destroyer (10 points) - Destroy 25 buildings during one game.
Juvenile Alien (10 points) - Complete the game on the Easy.
Adult Alien (15 points) - Complete the game
on the Medium difficulty.
Expert Alien (20 points) - Complete the game on the Hard.
Head Chomper (20 points) - Head bite 50 enemies without touching the ground or firing.
Killing Spree (15 points) - Kill 1,000 enemies in one game.
Mad Hatter (20 points) - Unlock the 31 hats.
Perfect Flight (30 points) - Defeat the final Boss without using a continue.
Survivalist (25 points) - Survive challenge mode for 20 minutes.
WMD (20 points) - Reach 5000 km in Super Soviet Missile Master.

-Select the Area 51 Challenge and start killing everything in sight. Wait for the big metal robot to appear from the right, but do not destroy it as it does not kill you. As soon as it enters the screen, stand directly under it and start shooting in the opposite direction. You will kill everything that appears from the left side and the enemies jumping over the robot with ease. Keep this up for about twenty minutes to easily unlock both achievements at the same time.

-For the"Survivalist" achievement, enter the Urban challenge and get in any car. Stay far to the left and charge your gun. Whenever you see an FBI car, release your charged burst and the car will instantly die. Recharge the gun every time you kill an FBI car. Do not worry about the FBI agents on the top of the cars; they are clueless and simply jump around.