Once you have played this game for the first time, you will be prepared to "play ball" in the Backyard Baseball League.
There are thirteen unlockables. How you can get them:
Abner Dubbleplay (get 20 DIFFERENT skill certificates)
Barry DeJay (enter coach's name as "hoops")
Shawn Green (get 60 career hits with one player)
Randy Johnson (get 75 career strikeouts with one player)
Pedro Martinez (pitch a shutout)
Derek Jeter (turn two double plays in one game)
Mike Piazza (hit a grand slam)
Greg Maddux (get 20 career wins with one player)
Mister Clanky (complete Clanky's Coaching Box with a season team)
Baseball Darts (get 40 points in the Fielder's Challenge w/ season team)
Quantum Field (win the Backyard World Series)
Aquadome (hit home runs into the water at three different bodies of water)
Humongous Memorial Stadium (make the playoffs)
To unlock the Aquadome, you must hit the baseball into the water at three different ballparks (Frazier Field, Gator Flats, and Steele Stadium).
When you start out, the three ballparks you can get for accomplishing certain tasks will be locked. Once you unlock them, you can play at them.