A self-sustaining, enclosed ecosystem within a miniature capsule to be viewed under a microscope. It would contain specific resilient species of microorganisms (tardigrades, etc) and everything they need to thrive.
This would be similar to the popular closed aquatic ecosystem - Ecosphere, where live shrimp and algae are enclosed in handblown glass. There is no exchange of air or nutrients between the enclosed ecosystem and the environment. In sub-optimal conditions, the tardigrades would go dormant.
Such a capsule could be built into a ring or a necklace pendant. Wear your pet tardigrade:)
It could be sent to Mars on the back/outside of a rover and periodically send a photo to earth so that we can keep up with how the tardigrades are doing in there.