Collect all the "KONG" letters in all levels on an island to unlock the special K level.
Successfully complete the special K level on all islands to unlock Secret Seclusion Island.
Successfully complete all three stages on Secret Seclusion Island to unlock the Hard difficulty.
Add a second player from the "World" menu before starting any level. Press Start, select the "Add/Drop" option, then press A on controller two. Select the control option, then choose to play as Diddy Kong, Dixie Kong, or Cranky Kong. Choose to remove the controller from the "Add/Drop" menu, and you will still retain your Kong Buddy.
There is a rock formation in the background of Level 3-1: Grassland Groove that resembles King K. Rool.