Mid to late summer, zinnias might be nice. The newer Zinderella Lilac comes first to mind, but there are lots of other pretty pastels as well. Several lavender transplants would give a bit of contrast to the rounded blooms of the zinnia, and neither requires a lot of additional water after they have settled-in. Watering your bulb bed during summer should be avoided, so plants that do well in drier soils are probably best.
