Complete the game to unlock Ability Planet (where you can grab any Copy ability) and Extra Planet Delta in the Far-Flung Starlight Heroes section in Story mode. Towards the end of the Extra Planet Delta level, you will reach a large background block with a cross and two dot platforms with an empty space above. A door will appear in the empty space. The background with the cross and dots is an original Game Boy. Go through the door to reach a hidden Game Boy style level. At the end of the Game Boy style level, you will reach a large button. Stomp on the button to summon a giant statue of Kirby, with the words "We [Heart] Kirby" over his head. If you are using the Switch Joy-Cons, the HD Rumble will play a lo-fi version of Kirby's theme from the original Kirby's Dream Land on the Game Boy.