Haven't had a chance to show thoses photos to Andy, but I already know what he would say.
"Build some forms and pour some concrete!"
Since that usually isn't a feasible option for those of us who work/play in our yards with whatever we can get our hands on, I go along with the others who say rocks. I am a rock FREAK! I have built rocks walls, steps, paths, borders, etc. all over this place. If I see a big rock on the side of the road I stop and get it!
(I saw some NICE ones in the background of your pictures!)