Access the following sub-levels by performing the corresponding tasks:
Turtle Woods Sub-level
The Turtle Woods bonus is located in level 1 (Turtle Woods). Belly flop onto the octagon with a menacing face.
Snow Biz Sub-level
The Snow Biz bonus, located in level 6 (Snow Biz), is opened when Crash acquires the Red Gem in level 2 (Snow Go).
The Sewer/Later bonus, located in level 12 (Sewer/Later), is opened when Crash acquires the Yellow Gem found in level 11 (Plant Food).
The Unbearable bonus is accessed in level 15 (Unbearable). When the first polar bear falls from the bridge and into the pit, follow him.
Cold Hard Crash! Sub-level
The Cold Hard Crash! bonus is accessed in level 18 (Cold Hard Crash!). Half way through the level, jump onto the object bearing a skull-n-crossbones tattoo.
The Ruination bonus, located in level 19 (Ruination) is accessible when Crash acquires the Green Gem in level 10 (The Eel Deal).
To find the red gem you must play level 7, Air Crash. When you come to the first jet board, instead of getting on it, jump on the first box you see and bounce off it onto another box. You will see a platform bounce onto it and you will warp to the secret warp room with a portal to level 2 Snow Go open.
Air Crash Gem
To get all the boxes in Air Crash, you must get to the Third warp room and play Level 13 Bear Down. At the end, when the bear throws you of its back over a frozen lake, return. Jump across the small tinted ice flows, and onto the large white one. You will warp to the secret warp room with the portal to Air Crash open.
To access level 26, Totally Bear, play level 15 Unbearable. At the end, the baby bear will toss you over a pit. Slide-jump back over the pit, and walk to where the bear is sitting. When you get there, you will go to the secret warp room with Totally Bear open.
To access level 27, Totally Fly, play level 16 Hangin' Out. When you fall into the water with the crystal, go backwards instead of forwards to fall into a hole. Play this area and you will warp to the secret warp room with Totally Fly open.
Revisiting Boss stages:
To revisit a boss stage, stand on the middle platform of a warp room, then hold the shoulder buttons (L1 + L2 + R1 + R2) and, when Crash points, press TRIANGLE, UP.
Go to the secret level in Unbearable. At about the halfway point is a bouncing box and an arc of fruit. Go to the highest step and super jump up to find two extra lives. Grab both of them and kill yourself, then return to that spot for 2 more lives. Repeat as many times as desired.