You could do a salvia in the middle and sedum or dicondria to spill down the sides. That would give it more interest and neither the sedum or dicondria take up much space in a pot.
Hey, think you got something there! Salvia will give colorful thrill, get the spill with dicentra. Btw, planters have southwestern theme. Color combo will pop. Thanks.
You're welcome!! The challenge is in the size so I poked around Bluestone's website to see what could possibly work. Creeping jenny would also be a good spiller.