Learn English – What’s the noun-form of “not working”

It's the difference between saying:

Having something to do has a lot of benefits for young people.

and

The act of doing something has a lot of benefits for young people.

The first corresponds to work, the second to working.

You already had the answer in your search term - noun will normally do fine as an adjective.

Consider, for example, a noun phrase, aka nominal phrase. So far as I'm concerned, noun there is an example of adjectivalisation (it's effectively being used as an adjective, the same as nominal).

In certain contexts you might use noun-like, but I'd avoid nouny, nounish, etc.