Here's something that's just plain fun...and may occasionally prove useful: "Ultimate Flash Face" and online "sketching" program that lets anyone draw a pretty darn good face.
I can personally testify that 8 year old boys love it...and that everyone who I've introduced to it has gotten sucked in.
The easiest thing to do is to try and draw each other's faces. It's a pretty shocking how accurate you can be if you're just patient. The next trick is to try on new hair or glasses. Arguing about whether or not your nose is that large or lips that thin can take up a fruitful half hour.
Advanced "students" will want to try their hand at caricature. Exaggerate Elton John's glasses. (Don't think it can be done? It can. The dimensioning tool is pretty nifty.) Play. A sick child or a grumpy friend can be nicely distracted.
The teacher in me feels compelled to point out that you can learn a lot about how crime sketches are composed (that process is clearly the basis) and about how faces are shaped. Have you ever really looked at other people's eyes? They're really different. Like all good instruction, you'll come away looking a the subject, in this case faces, differently.
What's wonderful, of course, is that this little tool is freely available on the web. Someone--actually a generous German by the name of Franks Fahrschule—has offered this up to the world. People all over the world get a bit of pleasure, a moment of fun, and the chance to learn something.
It's an amazing world we live in, really.