one that adds a virtual unicorn horn on your head, and rings will fall from the top, you move your head around trying to collect rings on your horn.
I would love to try it but some of the parents in our family won't let their kids on Facebook. I can't even post photos of their kids on Facebook. (They fun Flickr and feel they know too much about what happens to children online without great safeguards.)
But the kids love FaceTime and we all have iPhones, so stickers and Animojis galore. The parents are happy because end-to-end encryption.