Super Crown

The Super Crown is a power up that transforms Toadette into Peachette. It can seemingly only be used on Toadette.

Captain Toad: Treasure Tracker Special Episode
Captain Toad and Toadette decided to search for the Super Crown. This implies it was a famous artifact that was already known about. The crown is found by them in the Grand Labyrinth, and they take it back home.

New Super Mario Bros. U Deluxe
Toadette used the Super Crown to turn into Peachette on the journey to rescue Princess Peach.

Appearance
It is a gold crown with a pink and white mushroom cap. It also has eyes like most other power-ups.