Made on the same basis as lavender bags, fabric sachets are filled with activated charcoal (as used in home aquariums). These are supposed to absorb the sweat in trainers and shoes.
The sachets are lined with dark fabric as charcoal is very messy. I decided to label them with fabric pens so recipients know what to use them for and will enclose a note explaining that the charcoal can be reenergised if warmed in the oven.
Adapted from a Crafts Channel tutorial