Like, say, you are a revolutionary who’s imprisoned, but you are allowed to write letters to your family. How would you covertly send a message to your fellow revolutionaries that only they can decode? (Your family are on your side will pass the letter along)
Obviously, it cannot appear to contain any obvious ciphertext, since the prison guards would just rip up your letter and not send it.
I’d probably go with metaphors and callbacks.
“Do you remember that one time we went to the pier?” and make up a story or something that can be re-interpreted. IDK if that’s very good.