Have you considered the SmartThings Arrival Sensor? These are designed for use on a keychain, or in a backpack. They are Zigbee and simply send a periodic signal to hub. When the hub sees the signal, it updates the user's status to "present". When the signal is lost for a period of time, the user is marked "not present". You can then use these "presence sensors" to automate the arming or disarming of HSM, or to simply change the "mode" of the hub from Away to Home, for example.
As for buttons...
Here is a list of Hubitat button devices that are natively supported.
This post hasn't been updated in a while as there is now an official list of supported devices at List of Compatible Devices - Hubitat Documentation
