What does mid-height mean to you? Around 30" or so?

In my garden, I have white coneflowers that fit your description in terms of foliage and flowers both looking nice, but they are taller than 36". My purple coneflowers have faded quite a bit in the recent heat, but are still blooming and the foliage is fine.

I also have white Obedient Plant that is blooming and looks nice, but in some areas it is a very vigorous spreader, so you may want to read a lot of reviews before considering it.

Mid-height I'd say about 2', maybe 2.5" feet, that's probably the upper limit - can't do anything too tall, it will block the view from the picture window.

I have Tiger lilies, Echinacea, Phlox paniculata, Hydrangea arborescens, Hydrangea quercifolia, Rudbeckia, Sedum 'Autumn Joy, Gaillardia, Geranium 'Rozanne', and Clematis 'Betty Corning' in bloom. Lilium speciosum is about to bloom too.
This year all the lilies started to bloom earlier than usual.

I have added plants with interesting leaves to my garden. Hostas are doing great.

Forgot Echinops ritro. It's very pretty.

Balloonflower. Just keep it deadheaded, and it will bloom a good long time.

Definitely go with the Disco Belle hibiscuses! This is one of my favorite late summer cobinations - pale pink Disco Belle hibiscuses (and a darker pink seedling one from the pale parents...), Russian sage and veronicasturm. This is an old picture - Aug 19 2006 - but it's a combination that, every year, reliably puts on a good show starting in mid-late July and continuing to the end of August and beyond.
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Even just the Russian Sage and veronicastrum make a very attractive late summer combination that remains attractive as it fades:
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penstemons are your friends as are Salvia microphyllas or S.greggii. Grasses - stipas, anomathele lessoniana, heliotrichon, festucas, miscanthus or millium effusum - these will NEVER let you down, carrying on from early summer to flow effortlessly into later summer rudbeckias, crocosmias, asters and phlox. yep, hardy geraniums such as Russell Pritchard, Mavis Simpson, Patricia, Red Admiral, Brookside, Orion. Hemerocallis, especially daintier types such as Golden Chimes or Corky and lilioasphodelus, Perovskias, thalictrums, some of the later campanulas such as C.Lactiflora. As well as grasses, foliage is important too - artemisias, stachys, iris pallida, phormiums, astelia, rubus thibetana