I was in a yarn store a week or two ago and overheard someone asking for a pattern for a scarf. It was all I could do not to say, you don't need a pattern for a scarf! Cast on until it's wide enough, then knit until it's long enough! Done! But I was good.
Hats are slightly more complicated if you want a nice round bit at the top, but not so much that you really need a pattern for them either. If you've done a little knitting, and know how to knit, purl, cast on, etc., a patternless hat is well within your skill level. A bit of math and a swatch, and you're golden. Here's how.
Step 1: Tools & Materials
You'll need yarn and circular needles at a reasonable size, and some double-pointed in the same size (or close, no more than a size off in either direction). A tapestry needle is nearly but not quite necessary for hiding the yarn tails when you're done.
Depending on the particulars of your yarn, you will almost certainly need just one ball. Check the yardage on the ball so you know whether you need to adjust your calculations. My yarn came in balls of 100 meters.