Unusual construction using either a 3-needle cast-off or Kitchener stitch/grafting.
Simple to knit and an interesting introduction to the 3-needle cast-off if you’ve never done it before.
One size only, but going up a needle size would give a slightly larger hat without affecting the fabric.
1 skein/ball of standard DK would be plenty; this could be a good project for using up leftovers.