Nepeta, or Catmint is a very reliable filler perennial that does very well in full sun and somewhat moist soil. It gets thick, and almost shaggy looking, with blue/purple flowers. It looks great on it's own, along a perimeter or edge or combined with yellow (coreopsis, achillea).
Conditions: Full sun to part shade
Bloom: Early Summer -Fall
Size: 2-3' tall, 18"-24" wide