Keep your Fitbit Flex on your keychain, or around your neck! Your Fitbit snaps in tightly with a satisfying click. You can see the lights, and when the battery gets low, it can be pushed out through the back.
This version comes with a 6cm chain. The loop on the body is 4mm inside diameter, and the big rings are 8mm inside diameter, so it should fit on any keyring or pendant chain or string.
If you like the chain but don't have a Fitbit, check out this pen holder
Here's a video of the keychain in action: