I just picked up a book on perennial combinations. They suggest pairing globeflower with 'East Friesland' salvia. It also says, "Globeflowers bloom early in the season and often go dormant by midsummer. Plant a billowing fern or a bold-leaved rodgersia to fill the empty space. If you want more flowers instead, consider daylilies or turtlehead."

Hope that helps!