Any gift that suggests they need to improve something about themselves, especially if they’ve never shown any interest in that. Like a gift card for skincare treatments, or teeth whitening. Maybe a self-help book, or some exercise equipment. Cologne/perfume is good for deniability, but it might come off as more romantic than intended.

Also, giving any of these gifts to make someone feel bad about themselves makes you an enormous asshole. Use your words, be honest with people, and don’t go out of your way to humiliate or irritate people you don’t like. Life’s too short to spend it scheming.