Kirby's Epic Yarn

While you'll be able to unlock most of the soundtrack by finding the right pickups in the various levels, there are a few songs that you can't get without beating the game first. After you've defeated Yin-Yarn, you'll unlock the Dream Land, Yin-Yarn, and Staff Credit Theme music.

Several pieces of furniture are not available in the store by default and, instead, are unlocked by defeating specific bosses

There is a secret set of wallpaper that cannot be purchased at the fabric store. After completing all of the missions from a character in the apartment building, that character will offer to come over to Kirby's Pad after you complete a stage. Go back and visit Kirby's Pad and the character will be there, just as if they were visiting normally. However, after making their standard introduction, the character will present Kirby with a special wallpaper print. If you miss the character (for example, by going into a level before visiting Kirby's Pad), simply complete another few stages and the character will once again offer to visit Kirby's Pad.

The last cutscene you unlock also includes the game's staff credits. You'll need to beat the game first, and you can rewatch them any time in the Flicks menu.