These skinny beds are ubiquitous and always problematic. If you grow something sized to the planter, it is short, leaving the blank wall. If you grow something taller, it has to be cut back constantly to prevent interference with the walkway.

There seem to be lots of rocks in the bed -- why not purchase or collect some larger vertical rocks to serve as "natural sculpture" of sorts, and interplant with a few miniature evergreen plants? Something like dwarf juniperus squamata 'blue star'? Or a dwarf ornamental grass?

Clumping perennials like Heuchera. Use one variety for the bed. What is the plant with the small pink flowers? If it does well you could fill the bed with that.

A trellis with a vine?

I would consider widening the walk, especially at its ends. The wall & walk might be too confined for plants, or maybe just something uniform and fairly low. The wall might be better as a place to display art. Something along the lines of the material shown in this video might be a starting place for thinking about what might work there. Also, I've seen paintings of quilts outdoors as another possibility.

A low mounding groundcover would be my choice.

Amazing video, Yardvaark. I always wondered how they hand made tile. No wonder it's pricey!