Unlock Hidden Characters
These must be beaten in Championship Mode:

Mr. Garrison - Defeat Rally Days #2
Mr. Mackey - Defeat Spring Cleaning
Big Gay Al - Defeat Pink Lemonade
Mephesto - Defeat 4th of July
Grandpa - Defeat Halloween
Jesus - Defeat Christmas
Satan - Defeat New Year's Day
*Ned - Defeat 4th of July with Kyle
*Damien - Defeat Halloween with Kenny
Various Other:

Visitor(Alien) - Grab 2 Pot Pies on Memorial Day.
Ike - Grab the cheat item on the airplane wingtip on Memorial Day.
Terrance & Phillip - Grab 4 gold cows on Christmas.
Random Track Select - Defeat the entire Championship mode.