Question, if I have an unbreakable ring, what cool things could an engineer do with that?

Well it’s interesting that it makes regular ringing/metallic sounds when it’s dropped(at least in the movies), so it is not inelastic. Which means it could be deformed if placed under extreme force but it would always spring back into shape. So I think it might be the world’s most powerful spring.

is there something useful that can be done with just ONE unbreakable spring?

It probably produce/absorb heat somehow when being deformed. Then, we use it to boil water, turn a turbine, and generate power!