For security reasons, what should I block with the firewall? For security reasons, what should I block with the firewall?

I'm just interested in the system itself, not additional installed software. For example, I'd like my system to synchronize the clock or download system updates in the background.

Or, to turn the question around:

What on my computer would need incoming connections?

I don't use my Mac as a web server, but is there anything else that needs to receive incoming connections?
Well, basically the standard Firewall blocks incoming connections only. So as long as any network accesses are triggered by your Mac (as in the case of clock synchronization or software downloads/updates), you can block all incoming connections.

Once you have the firewall set up if you install anything that requires a connection you will be prompted to allow or deny that connection - so turning it on has little downside and lots of upside from a security standpoint.