The following actions are non-destructive:
Everything in the Layer > Adjustments menu.
These adjustments can be made to a single layer without affecting the underlying layers. This means that you can edit the adjustments or remove them entirely without altering the original image data.
Changing the Opacity of a layer
When you change the opacity of a layer, it simply reduces the visibility of that layer but the original image data is still retained.
The Clone Stamp tool, which is used to copy pixels from one area of an image to another, can be destructive because it overwrites the original pixels.
Converting to Grayscale also discards the color information, which is considered a destructive action.
Everything in the Adjustments panel is not a valid option because it includes both destructive and non-destructive adjustments.