Add cap blocks to the top row of a block or stone retaining wall. You can add a layer of masonry adhesive to hold them in place, then back fill. Or you can backfill as you go, then add the cap. Either way, it provides a nice finished look, much like edging added to a paver patio.
Native Violet, Viola hederaceae - I use it everywhere, with its bright green leaves and its sweet mauve/purple flowers raised above the foliage. It will rapidly fill in bare areas of soil between stepping stones.