Control main menu screen 

During the mainmenu, you can press Circle to "Ribbit" or press Square to eat bugs and fish

Level secrets

Collect every coin for a level to unlock the secret for that level

The following codes are all entered at the pause screen

All characters 

hold Square and press Left, Right, Left, Left, Left, Up, Left, Left

All extras and bonus levels 

hold Square and press Right, Up, Up, Down, Right, Down, Right. 

Infinite lives 

hold Square and press Down, Down, Up, Down, Right, Down, Up, Up
OR
press Right, Square, Triangle, Square, Triangle, X. 

Level Select

Hold Square and press Up, Down, Left, Right, Right, Right, , Down, Left.
OR
press Right, Square, Triangle, Square, Triangle, R1, L1, R1, L1, Triangle, Down. 

Level skip

hold Square and press Right, Left, Up Up, Up, Right, Left, Left. 

Temporary invincibility 

hold Square and press Right, Left, Right, Left, Up, Up, Left, Right.