I recommend stuffing with tissue or even soft toy stuffing and then sealing and wrapping in sticky plastic for durability if they are to be thrown but if they're going to be used as novelty gift boxes then just a simple black and white tag added with a black or even a brightly coloured ribbon to hold it in place might look fabulous. Or how about a pair of them tied together with ribbon as a real 1980's retro gift box. Could be perfect for bride and groom gifts for a wedding, engagement. or better still anniversary.
As I was flatpacking I added double sided tape for the customer to peel off and assemble following the You Tube video if necessary.
Sometimes the more simple a project is the more I like it.
Have a fun weekend. It's not far away now.