My brain said - they look mighty handy as containers for small balls of wool, for stranded colour knitting patterns, or for replacement warp threads, suitable weighted with coins, washers etc. Or kumihimo threads.
What about filling with water, adding food colouring. If there's a sunny window sill available, then use the coloured water to solar dye a small quantity of fleece or yarn, add a dash of citric acid or vinegar when pre-soaking the fibre (protein) It makes a nice experiment, regardless of the result. Plenty of info available on the web.