thumb prints made into Christmas lights. Easy craft to do with the kids. To enhance this, I might extend the strand each year, so by the time they are older, you have a long string of thumb print lights.
Christmas Eve gift box (they get to open it on Christmas Eve)! They get new pjs (to wear that night), a Christmas movie, hot chocolate, snacks for the movie, etc. We already do pjs and slippers as our Christmas Eve gift but this is even better!