Super Crown

The Super Crown is a power-up item that made its debut in New Super Mario Bros. U Deluxe. It is a puffy, mushroom-like pink headgear surrounded by a golden chaplet with eyes, exclusively used to transform Toadette into Peachette. The Super Crown's top appears to resemble the mushroom head of Toadette, being pink with white spots. When the crown is on top of Peachette's head, however, the crown itself transforms alongside her. The crown shrinks to fit her head, the eyes disappear, and the pattern on the top changes to four small white spots rather than two large white ones.

New Super Mario Bros. U Deluxe
The Super Crown can only be used by Toadette; other characters cannot collect it. If Toadette picks one up in a level, she transforms into Peachette, a Princess Peach lookalike that gives her the ability to Double Jump and perform Peach's Floating Jump from Super Mario Run, also controlling identically to the Flying Squirrel form (without the wall-clinging ability and a slower fall). Additionally, she automatically gets boosted out of pits, lava, or poison; however, if Peachette attempts the maneuver twice in a row, she will fall into the hazard and lose a life.

The Super Crown replaces all Super Acorns found in ? Blocks (but not the ones held by Waddlewings) if Toadette is by herself, otherwise only one Acorn is replaced. If Toadette is not being used, then the Super Crown does not appear. The Super Crown can also be won from Toad Houses and Enemy Courses by any character, but they cannot use it in the inventory.

Captain Toad: Treasure Tracker
The Super Crown reappears in Captain Toad: Treasure Tracker for the Nintendo Switch in version 1.2.0, where it is the main goal of the DLC Special Episode, and serves as an origin story for it. This game also introduces a more common, non-magical variant of the Super Crown known as the Shiny Crown. The Super Crown is located on the 30th floor of the Grand Labyrinth. After Chaos in the Grand Labyrinth is beaten, the entire episode is completed, and the Toads ride home in a minecart with the crown in hand. Notably, the Super Crown is a singular object in this game, and Toadette does not put the crown on to transform into Peachette.

Mario Kart Tour
The Super Crown reappears as Peachette's emblem and icon for the Peachette Cup in Mario Kart Tour, as well as on her head.

New Super Mario Bros. U Deluxe

 * North American website bio: "When Toadette finds one of these, she can transform into the super-powered Peachette. (Sorry Luigi-only Toadette can use this item!)"
 * European website bio: "An exclusive power-up when playing as Toadette. By collecting this, Toadette becomes Peachette, allowing her to double-jump, float slowly during freefall, and get a boost back up if she falls into a pit!"

Trivia

 * The concept of the Super Crown transforming Toadette into another character and granting her their abilities is similar to that of the caps in Super Mario 64 DS.