Finally, almost three years later, I came up with a way of incorporating planter boxes into the hillside, and using some other spaces, too.
The boxes were constructed from the leftover wood from our old stairs. I salvaged pieces that weren't too damaged.
The hardest part of the whole project was actually getting the open-backed boxes to fit on the hillside properly. It required a lot of digging, positioning, and more digging until I was finally able to get the box level.
Originally, I had planned for three planters, but due to the shape of the hillside, I ended up only creating two. The planter shown above is huge, so I think it will be the main, large vegetable box.
The front yard is starting to come together. Where the stairs used to be are now graduated planting areas sporting daffodils and the sprouts of wildflowers. Half of the wine barrel is going to be a planter for a small chaste tree. The other half will be a solar powered fountain. My veggie garden is finally underway, and the whole area should be mulched soon. Now I just need to get that clothesline built...