Draw faces on some balloons and let the kids loose with any old make-up you have lying around. Face paint should work too.
Given that I don’t wear make-up on a day to day basis (ok, or ever unless it’s a wedding or something – it’s too much like work), my kids have no idea what to do with it. But they had lots of fun smearing it over the balloons all the same.
It got a bit messy, but it wasn’t too hard to wipe up from the table and wash off the boys.