To unlock the What’s in the Box?! achievement in Little Nightmares 2, you’ll need to find and open the refrigerator on the Wilderness level/chapter one. It’s a very simple achievement to unlock, and if missed the first time around, you can use the game’s chapter select to easily pick up this achievement.
As you progress through the level, you’ll eventually come to the hunter’s house. Climb up and into the only accessible window, and you’ll find yourself in a kitchen. Simply run over to the fridge, jump up, and grab on to the handle with RT. The fridge door will swing open, and the achievement will unlock.