Hi Lynn,
If you don't mind a little work, your property is perfect for the "New American Garden". It's not really new anymore, but it's a term coined by James van Sweden and Wolfgang Oehme, Landscape Architects.
Basically, it is a low maintenance perennial garden. Many native perennials are used. Some examples would be Fountain Grass, Maiden Grass, Russian Sage, and Black-Eyed Susans. It is filled with texture and color and most important...a lot of fun.
The key is to group masses of one kind together, so that the outstanding feature of that particular plant makes a statement. Use a minimum of three, but even better, use five to seven of one type of perennial. Change the textures of those that are near each other.
In addition, you could have some paths meandering through the garden. The material can be as simple as loose stone, mulch or bluestone stepping stones.
Add a bench somewhere. Have some plants that hang over the retaining wall to soften it.
You would need to cut down the perennials once a year, and possibly do some dead-heading of spent flowers.
