For low-growing perennials you might look at:
Phlox
Snow in Summer
Sedum (groundcover varieties)
English Daisies
Perennial Dianthus

For Mid-Height Perennials, consider:
Bell Flower
Coreopsis (unlimited options)
Pyrethrum (Daisy)
Salvia
Scabiosa
Yarrow
Perennial Sunflower
Bee Balm (Monarda)
Sedum (Autumn Joy)
Iris

For taller perennials, maybe:
Balloon Flower
Delphinium
Tritoma (Red Hot Poker)
Perennial Hibiscus
Russian Sage (Perovskia)
Various Dwarf Grasses

I hope this helps!