Hidden Build Object:
A Leprechaun appears when you are building your own contraption on St. Patrick's Day. When he gets hit with a ball, he dances.
Hidden Part:
On Christmas Day a Santa Claus lamp will appear in the part bin. It works just the same as the lava lamp just shaped like Santa.
Secret Valentine's Day Object:
Play the game on Valentine's Day to unlock a Cupid part. It flies in a straight path like a blimp, but it can pop things with its arrow.