Tek-9 glider:

Complete the game (with any character) to unlock
the Tek-9 glider.


Hex:

Complete the game with all 3 main characters
(Zero-G, Chops and Sid) to unlock Hex as a
playable character.


Gex:

Ccomplete the game with all 3 default characters 
in time challenge, stunt challenge, and cash trial
modes with all medals.


Synder:
 
Get the time challenge goal for every level to 
unlock Synder.