Large faux fur pom-pom.
With an elastic loop and button fastening system - sew the button inside your hat, thread through the elastic loop (a crochet hook makes this a breeze) and slide over the button for a pom-pom that can be easily removed when you need to wash the hat. You can also sew on securely with a needle and yarn.
To fluff up your pom after shipping - use the cool setting on a hairdryer!