Backflip:
Jump off a ramp and hold Right/Up + C-Left. 

Stunts:

Nac Nac: Hold R and press Z
No Hands No Feet: Hold R and press Down, Z 
No Hands: Hold R and press Left/Right, Z 
Side Prone: Hold R and press Up, Z
Can Can: Hold R and press A
Recliner: Hold R and press Down, A
Cliffhanger: Hold R and press Left/Right, A 
Scorpion: Hold R and press Up, A
One Foot Can Can: Hold R and press B
Surfer: Hold R and press Down, B
Super Mac: Hold R and press Left/Right, B 
Rodeo: Hold R and press C-Down
Saran Wrap: Hold R and press Down, C-Down 
Toe clip: Hold R and press Up, C-Down
Fender grab: Hold R and press C-Left
Backflip: Hold R and press Up, C-Left