rednaz23:

You have the right idea going to a greenhouse or garden center. They can recommend some perennials that will do well in your zone. More than anything, you would want to know what colors your mother likes. Whatever you get should coordinate with the plants she has already in her garden, when they are in bloom. If you want to plant up that entire space for her, I recommend some tall plants in back, like lithrum (blue flowers) and something lower in front, perhaps day lilies which come in a variety of colors and maybe a little row of annuals in front, like alyssum, for instance. Any reputable garden center should be able to advise you.

Oh, and as an extra gift, maybe you and your mom could plant them toghether. I know I would love some of my kids to help me a little in the garden!