Introducing the Zam Hat!
Zam comes in three sizes to fit pretty much any older-child-through-adult head (normally I feel that hats are flexible enough not to need a lot of sizes, but extensive colorwork makes the fabric less stretchy and forgiving.) The colorwork is simpler than it looks, and is great for first-timers since you're only ever carrying a strand over two stitches (which means it's easier to keep the tension steady.) Malabrigo Worsted in a particularly forgiving yarn for colorwork, as well, and comes in a crazy range of colors for an endless number of possibilities. (Sidenote, I am LOVING stalking the Chevzam and Zam projects to see all the color combos. So much pretty!!)
You can check out Zam Hat on Ravelry here, or: