I designed a couple of coaster sets for my friend's birthday. I applied some vinyl decals to them to personalize the set. The inset face is big enough to accomodate any 4" (~100mm), or smaller, decal. Paper stickers will also work, but I'd recommend sealing them with resin or a few layers of clearcoat.

I also modeled a storage caddy for the coasters, to make out of wood, and then realized I could just print it.