See the ball of denim-blue yarn hiding behind the lilac one?
It's now a Slouchy Spiral Crochet Hat.
The instructions are pretty easy to follow, although, judging by the comments, some crocheters had trouble catching on to the spiral pattern. You're working in clusters: one front post double crochet (that's what makes the ridge), then three double crochets in the next space, even if there are technically two or three spaces before the next place to put a front post stitch. If you just made the crown, the centre part with spokes, you could have a coffee coaster; if you kept increasing for a couple more rows instead of starting to work your way straight down, it could be a hot mat for the table.
I didn't have quite enough yarn on the ball to make as many rows as required, and I could see that coming, so I stopped the clusters a bit early and started on the edge. It still works. I also discovered that somehow, in spite of thinking that I must own one of every size crochet hook made (I bought them all years and years ago), I do not own a 9mm hook. So I started with a 10mm for the first few rows, and then switched to a 7mm (but worked loosely). Having made more than one unintentionally outsize hat in my time, I decided that bigger was not better this time around, and took my chances with the smaller hook. Crochet can be very forgiving.