The Taupo hat is the perfect hat for all those 4ply/sock yarn leftovers. It works exceptionally well for that single skein you don't know what to do with, or with those part skeins for stripes!
Sized from Premature through to Large Adult, with instructions for earflaps, or without, this is going to become your GO TO pattern for lightweight hats for everyone you know.
The simple pattern is easy to knit, there are no seams, and it has a magically addictive quality, you'll be planning your second one before you finish the first!
I've also had success knitting this in Sport weight yarn, just check your gauge as you may need to go down a needle size.