You need to do this in a season for it to work, but you do not have to complete the season.
Unlockable-How to Unlock
Derrick Lee-Get 30 Season hits.
Paul Konerko-Hit a Grand Slam in a game.
Johan Santana-Pitch a shut-out against any team
Bobby Abreau-Earn 15 Seaon homeruns with one person.
Derek Jeter-Turn 2 Double Plays in One Game.
Dontrelle Willis-Earn 45 strikeouts during the season.
To unlock hidden fields the task must be accomplished in a season but the season doesn't have to be completed.
Unlockable-How To Unlock
Monsterous Stadium-Make the playoffs
Area 51-Make it to the World Series