I once put costume antennae on a headband and poked the antennae through the helmet vents and affixed two plastic eyeballs at the tips.
I then painted my face and exposed skin green, put on those slinky-loaded goo-goo eyeball eyeglasses, and went out for Halloween as a four-eyed space alien.
(Doesn’t even have to be a used helmet.)
I wear
both left and right Take-a-Look rearview mirrors (link), and once even put plastic eyeballs on them.