What services on macOS might leak identifiable information? I'm looking to make a set of firewall rules in macOS for use on untrusted networks, such that no identifiable information, such as account or device identifiers, might be inadvertently leaked.

macOS/Darwin not being my home operating system, I'm not sure which, if any, of the background services might leak such information in the cleartext. I'm particularly interested in cloudd, Handover, AirDrop and those services which would likely identify themselves to the network in some manner.

I've been unable to find existing guides for hardening macOS in this manner, but am happy to be directed to one if it exists.
I recently began researching issues related to hardening macOS 10.12. Fortunately I found a thorough "practical guide to securing macOS" on Github. The macOS Security and Privacy Guide (a 56-Page PDF) covers multiple macOS security topics, including application level firewalls; kernel level packet filtering (pfctl); background services (managed by launchd); system daemons; DNS variables; and many, many others. 

With regard to your specific question, this "hardening guide" should be a useful resource for an evaluation of background services that might leak PII or other sensitive data. (The section on Spotlight Suggestions was certainly an eye-opener for me). Finally: the guide concludes with links to dozens of "Additional Resources" and "Related Software" involved in hardening macOS.