I garden in similar weather and have been experimenting with rock garden plants the last few years. I have particularly been trying out a bunch of classic 'alpine' plants, which look great among rocks, but are often small stature - and many of them don't tolerate our summer heat & humidity. There are plenty of native plants which appreciate sun and well draining soils, which would do well also. They are likely to be less fussy. I bought a nice selection on-line at a missouri native plant nursery. They have done really well. Linaria purpurea and Asclepias tuberosa both arrived via some birdie - both do well in full sun and good draining soil. Barbara's buttons (Marshallia caespitosa), Amsonia hubrichtii, Pulsatilla vulgaris, Icelandic poppies, Silene virginica.
I second the recommendation of dwarf conifers - they look amazing in a rock garden.
But in my area, Abies, Tsuga and Chamaecyparus need a little afternoon shade to survive our summers.
Among the smaller alpine plants, among the ones doing well for me are:
Rock garden daphnes - there are many sizes. All flower beautifully w nice fragrance.
Creeping plants include thymes and veronicas, which are generally easy and nice spreaders. And many of the dianthus cultivars should thrive in your area.
Erysimum kotschyanum is another cute alpine creeper with bright yellow flowers.
Dracocephalum argunense & D. grandiflorum have grown well.
Alyssum oxycarpum is a flat creeper with grey foliage and yellow flowers.
Myosotis tergluoense and Houstonia caerulea have bright true blue flowers.
Aubretia gracilis has beautiful purple flowers.
Arabis x sturnii is a creeper with purple winter foliage.
Saponarias do well - several species available.
Horminum pyrenaicum.
Helianthenum is a shrubby plant which favors sharp drainage and has beautiful bright flowers.
I realize this is sort of a rambling list - I'm sure there are many others.
I suggest looking up these plants in the plant database to find something you like.
I buy most of my plants on-line since the variety is much better than you'll find at big box stores.
