Rise of the Guardians prompt (that I've been sitting on for a while but have done nothing with).
What if Jack quickly realized that while humans can't see him, animals can? He starts to regularly use a power that all seasonal spirits have (but that they rarely, if ever, use): shapeshifting.
When the other spirits think of the winter spirit known as Jack Frost, they don't think of the mischevious young boy with blue eyes and white, frosted hair--they think of the flighty winter spirit that is constantly shifting and changing between the forms of various animals--all who take on white fur, or scales, or skin, and blue eyes. Most spirits assume he doesn't have an original form.
Additionally: Jack has a handful of favorite animals forms, most of which are naturally white and live in almost constant winter conditions.
Additionally: Jack catches a glimpse of the Easter Bunny, and is quick to try to become whatever species he is. It quickly becomes one of his favorite forms.