When I bought my super expensive (but amazing!) pencil holder at Kinokuniya, I also bought a Zentangle kit that came with pens and artist trading card–sized Zentagle tiles. I don’t know if ATCs are really a thing outside the Zentangle world (a brief skim of the Wikipedia entry tells me not really) but it is a nice size to work in for Zentangles.
Here we have (from the top) Printemps (spiral things), Nekton (stacked alternating lines in groups of three), Static (nicely shaded zigzags, if I do say so myself), Knights Bridge (diamond checkerboard), Crescent Moon (half moon with radiating lines) and Nipa (spheres and wavy lines.)
I think I’ll send the finished ATCs along with my correspondence.