When it comes to newsletter referral rewards, operators often fixate on monetary value. But your audience didn't subscribe to your newsletter because they want free stuff. They subscribed because they want to hear from you.
That's why (exclusive) rewards which put your relationship with the audience first perform exceptionally well. Even when they're a bit silly and have zero monetary value.
Podcast (or video) bloopers are a great example of this. Give your biggest fans a sneak peek at what happens behind the scenes — and let them have a laugh at your expense too.