We have a solid tradition of drawing all over each other with sharpies at parties. (Is that toxic, by the way?)
Problem is, they only last a couple of days.
So, why not something a little more permanent, like henna?
The benefit is that would last a lot longer, like for weeks.
And while you're at it, make it more user-friendly than a paintbrush. You could have it available as a simple pen.
Oh, look, somebody already invented it.
This blog should be entitled "My awesome inventions that someone has already invented."