At the oversized LEGO box in Bricksburg, you can enter the codes below to unlock the corresponding pants for your character (which you otherwise must find and unlock over the course of the game).

Code	Effect
6LK78NN9 or HVLH63VL	Angry Kitty & Construction Pants
6LKMNDHR or HVLLRX6R	Blacktron Fan & Musical Pants
6LK3FRL6 or HVL4TQT4	Johnny Thunder & Super Secret Pants
6LK3RRY4 or HVL4TB94	Robo Pilot & Astro Pants