Candytuft (Iberis sempervirens) fits into the category of "looks delicate but is actually tough as nails." This evergreen subshrub grows about a foot tall, making it a wonderful edge plant. Its tiny leaves and profusion of small, white flower clusters that appear in late spring and continue sporadically through summer will soften retaining walls and pathways.
For a splash of color, look for a newer selection called Absolutely Amethyst. Learn more about candytuft.
Yarrow (Achillea spp.) has hundreds of cultivars available besides the familiar form with lacy foliage and golden flat-top blooms, each one with a different habit, hardiness, and color. All hold up to heat and drought, as well as deer and bunnies. Plus, butterflies love the flowers!
For summer, consider the cool-pink blooms of 'Cerise Queen' or 'Pink Grapefruit'. Learn more about yarrow.
Plumbago auriculata (also called leadwort) makes you feel cooler just by looking at it. This low, shrubby plant with small sky-blue flowers is grown as an evergreen in Zones 9-10, but works well as an annual in colder Zones.
There's also a pure-white flowering version that looks beautiful planted with anything.