Where in the world are you??? It makes a big difference :-)

Connecticut

Some ideas I had.

- Traditional groundcover: Ivy, etc

- red fescue

- A row of hydrangeas or hostas

The area below the pines is both shady and dry - pachysandra is the ideal groundcover. Evergreen, deer and rabbit resistant, drought tolerant and will grow in total shade to a fair amount of sun. Spreads pretty vigorously as well but not to the point where it takes over. Will need a bit of watering to get established but after that, can subsist on just rainfall.

Another option for those conditions would be Geranium macrorrhrizum.

Rhododendrons with pine straw mulch

How big do rhododendrons get?

Much too dry for rhododendrons, and the root competition would finish it.

Sold on pachysandra despite it being very common

Is there a taller plant with showy flowers that is shorter than the pine trees and can add some color and layering?

Did you check out Geranium macrorrhrizum? It is taller than Pachysandra, the foliage is very attractive, it flowers and has beautiful crimson tinged foliage in autumn.

I second G. maccrorrhizum. Lovely plant, great for dry shade, flowers in spring. And highly deer resistant and needs no attention whatsoever.