When people call something a public good, they usually refer to things like so-called public roads, public buildings, public schools etc.

But those things are not public. They're state-owned. You need permission to use them and you may even get arrested for trespassing f you try to enter a 'public' building without permission.
Calling them public is manipulative statespeak.

So what would it be fair to call public?

One thing is information. As a non-rival good, it's truly public.
This includes FOSS, but also any code that has been leaked to the public.
Similarly all the books (in the sense of content, not the physical material), scientific knowledge, musical compositions etc.
Protocols, like TCP/IP and Bitcoin.

I'd also say the internet, i.e. the network itself, is public. Its infrastructure (cables, servers, routers etc.) is owned by private entities, but the network is a public good.
As are natural languages (English, Spanish etc.), programming languages, sports, games like chess, the metric system and so on.

But what about the physical?

It gets tricky, because physical goods tend to be scarce.

One could make an argument that - at least from the perspective of our life on Earth - the sun, the moon and the stars are public goods; they benefit us (by giving us warmth, light or navigation tools), but none of us has any more right to them than anyone else, particularly due to our physical limitations, which don't allow us to access them in ways other than basking in their light or looking at them - at least for all practical reasons and for the time being.

The oceans and the air could also be argued to be public. The same limitations don't exist as in the case of celestial bodies, so their 'publicness' is not guaranteed. However, their public status is owed to a combination of factors: their perceived abundance, difficulty of access, low utility etc.

No one's land, such as the Antarctic, could be viewed as public, but if we find we can e.g. mine bitcoin there using geothermal energy, it will cease to be public in no time.