How do I save unlockables?

Looking for advice on the best way to go about this.

The idea is that a player finds a secret character or weapon and, upon exiting the game and returning, they still have said character or weapon. This would happen automatically without the player having to choose 'save' anywhere. Not saving progress.

I've found a few of the local storage high-score tutorials here, here, and here. Would that be an appropriate way to do this?

Yep that's the way. You can do ghost saves by just prompting a save whenever you want in the background.

— Thanks!