My best candidates:

• Urination / defecation.
• Vomiting.
• Popping pimples.
• Nose-picking.
• Menstrual hygiene (female-specific.)
• Visiting an urologist, especially to treat impotence (male-specific.) Edit: scratch that. "Not visiting an urologist to treat impotence" is definitely signaling, so this was an example of a negative-signaling activity (for the lack of a better term), and doesn't belong here.

To a lesser degree:

• Brushing one's teeth (though "high-quality" dental hygiene habits such as daily flossing can be used to signal that "I always have a fresh breath, so come kiss me!".)
• A solitary walk in a park / nature (can be used to signal that "I dislike crowded places" and "I am of contemplative nature".)