A New Years Tree! Such a great tradition to start. Put money/jokes/fortunes in balloons on the Christmas tree sans Christmas ornaments (only lights). At midnight let the kids pop the balloons and they get to keep what's inside.
Every January turn all of the hangers in your closet backwards, like this. It takes about 60 seconds: Each time you wear an item, hang it back up in the normal fashion, which takes zero effort to remember or do. In June, any item that is still hanging backwards has not been worn in 6 months, donate it. @Rachel Marchant