You can use shrubs at the back of the border, or tall ornamental grasses. In shady areas you can use some of the larger ferns for height. Think not just height but texture. Most ferns or grasses have relatively fine foliage, but their shapes are different enough to create a pleasing contrast. Then you may want to look for a couple of plants with big, bold leaves to create structural interest.
Some leaves are very intricate - like most ferns.
Some are quite simple, like the oval, slightly pointed leaves of the dogwood. Others are tiny and quite dainty - like the many nepetas and campanulas, or Veronica peduncularis 'Georgia Blue'. Then you have the spiky ones - yuccas, bearded irises and kniphofia to name a few. Many grasses grow erectly, but others flow outward in a graceful vase or fountain shape. Some form perfectly rounded mounds.