How about creeping phlox? It would only drape over a bit, but would certainly soften the top edge.

Sweet woodruff and Lamium are also suited for trailing.

Are you planting veggies in the areas of the walls? How about trailing/flopping herbs, or vining veggies?

How about herbs? Oregano would be nice, and there are some decorative types. There's a cascading hardy rosemary, too.
Don't plant mint, though. It'll take over.

Marty

The things that I think look the best are also things that are moderately slow to fill in. Creeping phlox is indeed a good choice, but takes a couple of years to look established.

Many of the herbs, especially those that trail grow fast but can be very invasive. Some creeping thymes would work.

Two things that I particularly like are the wonderful scent and silvery foliage of lavendar--- while slow to get established it loves the hot sunny environment that a wall usually provides and is drought tolerant. Many things will look fine early in the season only to look beaten in late summer and early fall. The other is artemesia. Powis castle grows as a fully silver lacey mound with a brush by fragrance that I like, but not everyone does. There are other varieties with different leaf textures for interest. Lavender has wonderful scents that almost everyone likes.

Carolina jasmine smells lovely in late winter and early spring but can be slightly invasive over several years..and can get a little ratty late in the summer.

I'd use a combination of things to create a wider variety of interest over a longer period of time.

If you don't have the patience to wait a year or two for slower growing things to fill in, you might try using something that grows quickly like creeping jenny or some of the more ornamental ivys that are very invasive. With careful attention they can be controlled by submerging a potted plant inside a slightly larger pot submerged in the soil.(the straight-sided black plastic nursery pots)

I also like sedums of which there are probably thousands. They're generally fast growing, drought tolerant, offer textural variety and a few will grow thickly enough to keep grass from overtaking too quickly. A few are evergreen and offer some interest even in winter though most are not.

If your wall has a flat top surface, potted plants evenly spaced could make it look less harsh....and give you some immediate interest.

Thanks everyone. Lots of good advice.

Jason

Is there any issue with the root structure "pushing" a retaining wall?

Our wall is as high as 8' in some places and keeps the backyard in the backyard!

We've thought of plants that cascade down the wall, but wonder if the plants can endanger the wall in any way?

Thanks!
Cynthia