It all depends on which game you have... I remember that to get to level 31 in crash Bandicoot 3: Warped, you need to get caught by the second Pteridactyl in the group of them near the end of level four. I'm not certain of this, so you should check it just in case.
I've got two:
-Crash Bandicoot:Warped, on one of the motorcycle levels there is a yellow sign with an alien face on it. Crash the bike into it and you will be transported to a secret level you can't access anywhere else
-In Crash Bandicoot:The Wrath of Cortex, start a new profile and type your name to be 'Wombat'. When the cutscene finishes, you will have unlocked everything, even the secret platform in the middle.
That's all I know.