My daughter is obsessed with Minecraft and asked me if I'd make her some bookmarks, so I made a Creeper.  Fairly simplistic design, it does have a slight 3D texture to the top surface and is printed so that the face is visible on both sides.  You may want to scale it up or down depending on preference, I made it so it'd be suitable for smaller books but works with larger ones as well.

I would strongly suggest printing these in bulk and/or adding a purge item just so that you're not wasting filament on only one or two bookmarks.