Honestly, I’d be wary of using your own writing as a free gift unless that writing is already for sale and it’s a limited promotion. Your work is a product and you need to treat it as such. People should be signing up for your newsletter because they’ve already read something and are hoping to pay for more. Don’t cheapen your work.
I mentioned print quality digital poster above. An even better choice might be an original art badge with a quote that people can use in signatures and websites and such.
That's an interesting response, because there are a few self-publishing "experts" who are suggesting a freebie short story is one of the best reader magnets to induce readers to sign up. And I'm signed up to a few author mailing lists who do just that. Though if I'm honest, I'm not sure I'd pay money to read those stories anyway, as they generally serve to explain more about events or characters I've already read about, that might lack the power of a real standalone short story.