Rib-A-Roni hat (and another with a green stripe and brown background) with some teen-aged nephews in mind. As you can see, it fits my 8-year-old son just fine, too. I made it in the Adult Medium size, but I had read other people's comments that little kids' heads are a lot bigger than we think, and I tend to agree! There's plenty of stretch left to fit a bigger head, but it looks fine on my son, too.
Are you knitting or crocheting any Christmas gifts this year? I'd love to see them!