I am not sure if you have a sunny, shaded or partial shaded area...I am assuming sunny since you have daylilies. I had good luck with blackeyed susans rudbeckia (sp?), daisies and coneflowers. Bee balm also goes balistic LOL.

Right now I have mostly shade and partial shade and love my hostas, bleeding hearts and large sedums. Oh another quick border plant is snow on the mountain, but it depends on if you want this to be the only plant. It lookes pretty but is extremely invasive! Oh yeah, ribbon grass/zebra grass...pretty much same as SOTM.

If you happen to post looking for something that grows to quickly fill in areas make sure you look to see how invasive it truly is before planting. I once planted a single small tuber of jerasulem artichoke aka sunchoke and have been fighting them off ever since...they looked cool at first but were so invasive where I planted them I considered Round-Up for the entire very large bed to try to kill them off.