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.