Enter these codes at the games Cheat Menu:

Code Effect

PRPPAPLYR Thin players

GOTHELIUM Big heads, bats, and feet

BIGHELIUM Big heads and feet

ABBTNCSTLO Fat & Skinny mode. Makes some players very skinny and others very large.

BBNSTRDS Big Baseball mode. Makes the ball really big!

GRTBLSFDST Smoke Trail mode. Baseball leaves a trail of smoke behind it.

LIZARD TRICK:
While playing in a normal game, in Kaufmann stadium (Kansas City Royals), there are two signs that say "Win a Lizard." If the player hits one of those signs while batting, his team will turn into lizards.

Alien Abductionn Mode:
ATEMYBUIK
After entering the code, start an Exhibition or Quick Play, select a stadium, and scroll to the bottom to "Alienapolis Park." The home team will have the same stats as the team that was picked during set up, but the players will now look like aliens.
Thanks to Chris