Unlock Baby Crash Costume:
     Get all the Die-O-Ramas

Getting Skeleton Crash costume 
     Speak  with  the  Astro  Land worker. Give to him 4000 coins. For
exchange, you will get the Skeleton Crash costume

Better weapons:
You can buy these At Midway 
Spend 500 coins - Second level weapons
Spend one Power Crystal - Third level weapons

Unlock new Characters:
Give Crunch 2500 coins at Midway - Unlock Crunch Bandicoot
Give N. Gin the Green Plutonium at Midway - Unlock N. Gin
Give Nina the keys at Midway - Unlock Nina Cortex
Give Pasadena one Power Crystal at Midway - Unlock Pasadena Opossum
Give Von Clutch one Power Crystal at Midway - Unlock Von Clutch

Earn coins easily:
     After  you  drink  a Whumpa Whip, let a Ninja Penguin attack you.
Then, just pick up the coins.