Revisit Boss stages
Stand on the middle platform of a warp room. Then, hold L1 + R1 + L2 + R2 + Triangle and press Up when Crash points up or down. Note: You only can return to Bosses you have previously encountered.

Bonus voodoo mask
After losing a life, hold Up + Circle until Crash moves again. Alternatively, while in a warp tunnel, press X(2), then hold X + Up. Release the buttons when Crash receives a voodoo mask after the next life is lost.

Extra levels
In the level "Unbearable", when the second polar bear falls off the bridge. Jump in after him to enter a bonus level.

In the level "Turtle Woods", after the "Check Point" box is an octagon on the ground with a menacing face. Do a Belly-flop on the face and Crash will fall through it into a new level.

Extra lives
In the second warp room (level 6-10), jump on the baby polar bear's head to collect ten extra lives.