Maybe for certfain holidays the background could be changed to tie in with that holiday?
Example:
Easter: Easter eggs scattered around the background
Halloween: Sonic is replaced with Werehog, Shadow is holding a gun, Tails is peplaced with Tails Doll
Christmas: All characters wearing Santa hats, background is a snow related level in the Sonic franchise (Ice Cap for example), with fireworks in the sky (New Year may aswell be thrown in there).