What an inviting starting point you have to work with! I love the beautiful stone walls and terracing. I would suggest adding some shrubs (to add bones for the garden) in addition to the perennials Marie suggested.
I personally love rhododendrons and have had the best success with 'Helsinki University' and 'Scintillation' in my windy, hilltop zone 6b gardens. Both have pink flowers.
The fallen leaves dropped by nearby trees can remain in place under rhododendrons, no cleanup needed, as in a natural woodland setting. The leaves help to cool the roots and maintain moisture in the soil, as might be desirable in your raised (terraced) garden. I think that rhododendrons would make a nice backdrop for the hostas and other plants Marie suggested.
Here is an example from my garden:
The hostas in the photo are 'Paul's Glory' and the plant on the far right is a Siberian iris, which might be an easy-care choice for you too.
