Successfully complete Story mode by beating all the pro challenges and Tony Hawk's final challenge. You can now buy any of the following cheat options in the Skate Lounge for $1,000 each:

First Person Cam: Skate in first person view
Giant Mode: Huge skaters
Tiny Mode: Tiny skaters
Matrix Mode: Slow down time during ollies/air
Super Jump: Ollie higher, except off halfpipes and quarterpipes
Mirror Mode: Mirrored levels
Unrestrained Aggro: Easier timing for Aggro Kicks
Perfect Balance: Perfect Rail/Manual/Lip balance
Replay Camera: Skate with replay camera
Ice Mode: Visual: Bluish levels
Night Mode: Visual: Skate at night
Old Timey Mode: Visual: Film overlay graphics
World Of Darkness: Visual: Darker than night
Steel World: Visual: Dark graphics
Light Inversion: Visual: Shadows turn to light

Enter the Skate Lounge, and select "Change Gear" to access the following skaters after they are unlocked and purchased for $2,000.

Hippie
Santa Clause
Easter Bunny
Zombie
Robot
Ninja