pds, maybe it was a Catholic grade school thing or a my-mom thing (can't remember). At Christmas time we would wrap an empty box with Christmas paper, and would give up stuff (the sacrifice part) or do good deeds that would invisibly go into the sacrifice box. Also can't remember what we did with them after Christmas, but we felt incredibly virtuous.
edited: I finally remembered what we did with them. Gave them to Baby Jesus as presents under the tree. I think the nuns put us up to this.