Sunglasses

Pure vanity, with one joke attached. While you have them on, the sun in the sky swaps to an older cartoon version of itself that is also wearing shades. It is a background change, so you will notice it in an open surface build and never underground.

The effect leaks past the world too. Quit to the menu while wearing them and the title screen sun keeps its sunglasses until you load into a world where you are not. If you keep a Cattiva around, it puts on a pair as well.