Is it a contained site–not near streams or waterways?
Polygonums, Loosestrifes (Lysimachia) and mints, but if near water roots can spread downstream making them invasive. Vaccinium or lowbush blueberry might be a good choice if a more woody perennial is desired. Osmunda ferns and Siberian Iris are more tall and clumping, but placed close they provide a nice mass to cover the ground.