Thanks for writing. Most perennials (plants that die back to the ground every winter and come back from the roots) tend to have small root systems, so you should be able to plant pretty much anything you like and that is adapted to your area. You can even plant some small shrubs (varieties that don't grow more than 4 or 5 feet at maturity, for example).
You might check out our collection of free garden plans for more ideas --- you might even find a whole garden you like! http://www.bhg.com/gardening/plans/