Many geocachers have Signature items that they like to leave behind in a geocache.
Some examples of these are:
- Custom Stamps
- Small animals (relevant to Cacher name)
- Cacher Cards
- Custom clothing
These are by no means compulsory, but many cachers enjoy adding this touch of personalisation when they hide/find a cache.
Signature items to into a cache as, or in addition to, normal swag.
This US Webpage has many, many examples of signature 'coin' type items.