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.
Use your front yard's slope for a decor advantage! More pretty front doors: http://www.bhg.com/home-improvement/exteriors/curb-appeal/curb-appeal-on-a-dime/?socsrc=bhgpin072314bringstyletoaslope&page=19