Level select:
At the Main screen, go over to the ant hill. Now hold C-Up + C-Down + C-Left + C-Right + Z and press R. You’ll see an arrow to confirm entry.

Invincibility:
During the level description hold Z + R + L. Flik will be invincible the whole level. This only works for one level! You may have to enter this code every level to be invincible the whole game!

GameShark Codes

Infinite Health811e1a2e0004
Infinite Lives801e1a380009
Max Grain    801e1a390032

Have All FLIK801e1a3a000f
Start with Goldberry811e1a2e0003
                        
All Enemies Killed811e1a260000
Always Have Super Jump811e1a280020

Unlock All Levels81099150000f

Thanks to Revolution readers Randolph S. Remoroza, Isaac Britton and Rickety!