Unlockable skins:
Unlock the following skins by entering the corresponding codes at "Extras" > "Unlock Content". Then, select "Customize" at the options menu to activate the unlocked skins:

60's skins (crypto) - M13Ni95L
70's skins (crypto) - S63bf2kd
Biker skins (crypto) - 1gb57M2x
Chef skins (crypto) - 51c24KiW
Gangster skins (crypto) - J5d99bPz
Kung fu skins (crypto) - Ly11r98H
Mime skins (crypto) - 7qd33J1n
Saucer attatchments - V81fvUW3
Saucer skins - X91mw7zp
Velvet skins (crypto) - F9sT5v88

Completion bonuses:
Beat the game to get 3 million DNA and ammo recharge for your non-selected weapons.

Destroy all humans ad:
Go to the space dust in las paradiso. If you look closely at the movie posters there is an ad for the original destroy all humans.