Hidden Hats
In the Options Menu, where you can rename your alien, use the following names to unlock hidden hats for the 2 player game:

    * cletus
    * april
    * superfly
    * goodman
    * grrl
    * abe
    * princess
    * dandy 

Head Gear

    * Area 51 Hat: Get to level 3-1.
    * Blond Hair: Beat the final boss using hard difficulty.
    * Diamond Tierra: Beat Level 2-2 using hard difficulty.
    * Jouster Hat: Beat Level 3-4 using hard difficulty.
    * Private Eye Hat: Beat Level 3-2 using hard difficulty.
    * Soviet Union Hat: Get to level 2-1. 

Super Soviet Missile Master mini-game
Finish level 1-5 and the Super Soviet Missile Master mini-game will be unlocked after the intermission sequence.

Minigames, Levels and Hats
Set the player 1 name to "ROYGBIV"