My favorite thing to fill in small spaces is individual packs of Kleenex and indiv. bags of hot chocolate, Kool Aid, Etc..
Very lightweight, it can squish into odd shaped spaces, it cushions, and they can always use it!
I also slip lip balm into tiny spaces. If contents are individually wrapped (like hard candy) and it won't affect freshness I also *** bags to smoosh all the air out to save space.
Mrs. A