Cheat menu:

Press L, R, L, R at the pre-game menu. Note: This code must be
activated before the other pre-game menu cheats are enabled. 

Devil's Thumb stadium:

Press Right, A, C-Up, L, A at the pre-game menu. 

Always hit home runs:

Press L, A, Down, Right at the pre-game menu 

Crazy ball:

Press C-Right, A, Z, B, A, L(2) at the pre-game menu. 

Crazy pitches:

Press C-Right, A, Z, C-Up, R, B at the pre-game menu. 

Varied pitches:

Press C-Right, A, Z, C-Up, R, L at the pre-game menu. 

Easy steals:

Press C-Left, A, Down, C-Up, Z at the pre-game menu. 

Aluminum bats:

Press R, A, Z, B, A, L(2) at the pre-game menu. 

Red bats:

Press R, Down, B, A, Right at the pre-game menu. 

Blue bats:

Press B, L, B, A, Right at the pre-game menu. 

Psychedelic bats:

Press Z, B, R, A at the pre-game menu. 

Low gravity:

Press Up, R, A, L at the pre-game menu. 

Increased gravity:

Press Up, Down, L, Up, R at the pre-game menu. 

Fast game:

Press L, A, Z, R, B, A, L(2) at the pre-game menu. 

Slow game:

Press Up, L(2), B, A, L(2) at the pre-game menu. 

Alternate sky:

Press C-Right, A, Z, C-Up, L, R, Z at the pre-game menu. 

Bonus teams:

Press C-Right, A, Down, Left at the title screen. 

Hidden message:

Press C-Up, R, B(2) at the title screen. 

View credits:

Press R, A, Z, R, C-Right, A, B at the title screen.