Not the parent, but the obvious answer is: a hard token (e.g. Yubikey). After all passkeys are just a software emulation of the smart card / FIDO2 mechanism that's been around for many years.
The issue with hard tokens is that there is only one of them. By design, you can't back up a Yubikey's content to a second token. This means that any time you add 2FA to a new account, you must have all of your hard tokens in your possession to enroll them. This means a "one token on your keyring for daily use, one token in a safety deposit box as backup" approach isn't possible.
Yubico did propose a potential solution five years ago[0], but that proposal seems to have gone nowhere. Until something like that gets implemented, FIDO2 (and by extension Passkeys) requires some form software implementation backed by cloud synchronization to actually be usable for the average person.
It works well enough. When you need to signup for a new service on the go, you can add your backup key when you get to it. Having the backup key in a safety deposit box hardly accessible seems like a non-goal given you protect it with a pin with a very limited number of retries.
> When you need to signup for a new service on the go, you can add your backup key when you get to it
Good on paper, bad in practice.
Requires you to remember doing that each and every time. Incidentally this isn't that different from just grabbing your keys like the parent suggested. Only it introduces a new variable: time delay. A lot can happen in that time and we all know the reality is that even a diligent person is going to slip now and then. It surely isn't a reasonable expectation for an average person.
