What gorgeous pots! I have never seen anything like them! If you want whimsical, Mexican hair grass would be good: Stipa tenuissima. :)

Sage, horehound, and thyme all like good drainage and would probably return next year.

I think it should definitely be an herb that drapes a bit, or ivy.

beautiful pots! How about creeping jenny or sedum 'Angelina'? both very hardy (almost indestructible), w/ trailing foliage...

what about something like black mondo grass for the king? or something equally tidy looking, a matt forming purple sedum? And for the queen that trailing silver dichondra? Cool pots!

Beautiful pots!! I second the suggestions for thyme, maybe the variegated yellowy green thyme or with the purple sedum? In my experience, three things you can't kill are thyme, sedum and creeping jenny. And again, LOVE The pots.

Marcy

I would plant Oregano 'Kent Beauty' in the Queen and Lotus berthelotii in the King. :)

Oh, these are awesome! I am sure we all would LOVE to know what you chose to plant in them!
I think I'd have gone with a sedum, especially 'Blue Spruce' as I think it would echo nicely the verdigris in the crannies of the terra cotta.
Sedum 'Golden Acre' grows really fast and is perfectly drought tolerant/resistant, but not sure the color would be as complementary.
Hope you post a pic!