TinkerKat-
I love perennials for the obvious reason - there they are, year after year, bigger and better than the year before. The frustrating part is finding a perennial that blooms most of the summer. If you want flowers, many perennials will only bloom for a couple weeks (at best) during the summer. Daylillies, for example, frustrate the heck out of me..."oh, how pretty, they're blooming...wait, where'd they go???"
But I have a good one for full sun, lots of color, good border plant, and summer-long lasting flowers. It's Coreopsis. I'm still learning about it, but have had a lot of luck with it since I have a lot of hot sun also. Most, I think (anyone correct me if I'm wrong) are low-growing and ferny looking. It blooms tons of flowers and continues to bloom throughout the season. Some varieties are taller than others. The colors I have are yellow and purple, but there may be more. I will find out more about what's available since I intend to fill a bed with these this year.
And, in true perennial style, they get bigger every year. However, unlike some of my other perennials, I had luck with them the very first year planted. Sometimes you have to wait a year for things to take root and get going.
If I learn more I'll let you know, but I think this one's perfect for you. BTW, it's pronounced (so you can look smart when you ask about it!) KOR-EE-OP-SIS.