Another way to do it would be to feature their profiles on the front page of the site. We kinda do this with badges.p2pu.org, but we have yet to implement a curation aspect to feature the best ones.
I’m actually less a fan of merch/swag and more a fan of social capital. I think shining a light on folks we love will encourage them to keep engaging.
We could also do zany, offbeat things. We could make them a thank-you playlist on Spotify where we all contribute one track. We could record our personal thanks on SoundCloud. We could send them a user-generated some(e)card. These are just a few ideas.