This is a really easy way to get Marvin from the Hitchhiker's Guide to the Galaxy up on your tree. Still trying to figure out how to get a lovely voice clip of Alan Rickman to go along with it, but we can't have everything.
The material list is fairly small:
Christmas bauble (go for frosty white or silver)
Black permanent marker
Using an elastic band to act as the black eye line guide, draw a line across the bauble with permanent marker. It's a bit tricky to keep a steady hand, but I found that the elastic helped a ton. Make sure the marker dries completely before you remove the band. The top part of the line, where it rested against the elastic, will have a bit of a rough edge, so try to carefully cover it with the marker again.