One of my favorite plants is yarrow, and it should work well since you get so much sun. As long as the site isn’t wet – yarrow likes it on the dry side. It comes in a wide range of colors, and has a very companionable vibe. It will die back to the ground in the winter, but even under 3 feet of snow you can find a few green yarrow fronds hanging out waiting for spring. I planted Veronica “Sunny Border Blue” two years ago, and have been amazed at how well it’s done. The flowers persist all summer, plant dies back to the ground in winter, and is up again in the spring. Salvias are also very easy to live with. For annuals, zinnias and marigolds can’t be beat, and petunias as long as you keep them well watered.
As for when to plant, the last frost date in these parts is mid-April I believe, but if Home Depot and Lowes are selling them, you can probably plant them without too many worries.