On Revenge of the King, if you go to Stage 2, there is a secret room with the word HAL spelled out. To find it, go to the area with the stone switch. When you hit the switch, and the blocks break, go to the left and press up. There should be a door there. When you go through, you will face a mini-boss. Beat him and continue to the right, Soon, you should get to a room with blocks spelling out HAL, along with an Invinsible Candy.
When you are battling the final boss of Spring Breeze (King Dedede), if you go to the far left and right of the arena, you can spot characters from Mario games.
Throughout the course of the game you can unlock little films in the Theater mode where you can watch hidden videos in the game or the past cutscenes. Below is the film name and how to unlock them.
Kirby Super Star Ultra EASTER EGGS
3279
