Daylilies as you see are very colorful and very pretty. They are a fast growing plant and you can usually split the plants at the end of each growing season to use elsewhere in your flower beds. Just be sure to "dead head" pull off the old blooms when they are dead to give the plant a chance to give nutrients to other parts.
Peonies are very easy to grow and come in various colors to compliment your flower beds. Just be very careful when "smelling the flowers" as ants tend to like these flowers a lot.
Rhodendrons are easy to grow, but as you can see by the picture above, they will bear trimming occasionally as they do tend to get really large.
Ferns are very easy to grow and will spread after the first season. Ferns love the shade but will do well in partial shade.