How about a few badges as a rememberance to a Winter Bash?

Such as:

* Bronze: Get a normal hat on a site. (awarded once per Winter Bash per site, not once per hat)
* Bronze: Get a secret hat on a site. (awarded once per Winter Bash per site, not once per hat)
* Silver: (**Collector**) Get ten hats on a site.
* Gold: Get all normal hats on a site.
* Gold: Win the winter bash. The badge will be awarded to all sites you have at least 1 hat.

Hats will disappear, but then you have at least a few badges as a rememberance and a symbol to the nice Winter Bash.