If you want to retain the mulch and so forth from the sidewalk just about everything DIY is going to stand up above the sidewalk. You might look into the cost of the poured landscape borders. The company would give you a border that was flush with the sidewalk to start but sloped up and away from it to retain the bed contents. Border Magic is one such franchise but there are others. They offer a variety of colors and patterns.
http://www.bordermagic.com/GallaryPage1.htm
http://www.bordermagic.com/GallaryPage1.htm