I am thinking about... an exclusive simple item made specially as a prize. Item given out whenever the person reach certain post level?
Like, 100th post. They get a black color of the item.
and then 500th post. they get red color of the item.
... so on. Same item, different color.

I would think the giving part is a lot of work?

I am a hoarder and collector. <33 I love to collect stuff like those.