Eryngium, better known as sea holly, has bluish-purple global heads surrounded by stiff bracts, which make them an unusual addition to flower arrangements. They command attention and add interest to your garden. I saw them planted next to rudbeckia (black-eyed Susan's) at Fuller Gardens and it made a spectacular display.
Another lesser known perennial is eupatorium. Joe Pye weed grows to 3-4 feet tall in full sun and bloom with clusters of white flowers in late summer or early fall. I have the "Chocolate" variety which has deep purple-brown leaves-- a great contrast to all the green in your garden.
If you don't already grow these three E's, I encourage you to give them a try.