What can a publisher offer to its best reviewers?

There are a number of quite reputable journals that actually do give public "outstanding reviewer" awards, and their value (while not large) is non-trivial.

If the journal announces their awards publicly online, as these tend to do, then there's no worry about faking a certificate or such. It's probably not going to win anybody a job, but it's another line on the "service" or "awards" portion of a person's CV, and it will be noticed in their community.

Actually, I'd be happy just to get some feedback from the editor about the quality of my review. Doesn't have to be every time, but I would really appreciate getting comments when I do a particularly helpful one and what was good about it. Similarly, I think Publons now has a 'rate this review' feature that allows editors and/or authors to score or give comments (not really sure, I haven't used the feature).

One of the journals I occasionally review for provides something like a 150 USD voucher for their books. This is given for every review or for everyone who reviewed at least once or similar. Not sure of the details as I haven't reviewed for that journal recently, but I definitely remember appreciating getting a book I otherwise would not have bought.

• A variant on "free book" is "store credit with the publisher". This gives you the flexibility to offer a smaller amount. Or it can be something like "X% discount on purchases from the publisher."

• If there is a prominent conference in your field that many of your reviewers attend, you can have a special reception at the conference, with free food and drinks. This may also be a good way to make connections with them and get feedback.