For shrub gardens under deciduous trees - nothing beats the simplicity of the good old fashioned Viola - Violets. The glossy heart shaped leaves just look so lush 12 months of the year, with the added bonus of the very fragrant tiny flowers in winter and spring. I don;t know what you you buy them as these days (maybe Viola odorata?) - I have always just yanked mine out of my own or someone else's garden, and poked them in the ground. They can be a little invasive of smaller plants - so let them fill a large area between shrubs is best. Easy!
The tiny flowers are gorgeous in a delicate little vase, with a few leaves poked in to surround them. Very fragrant. They come in many shades, but the strongest seem to be purple and white. They are a generational plant, too. I have some that were my grandmothers, and my mother-in-law has probably 20 variations in colour, with shades from salmon pink to bright pink, white and various purples. Gorgeous as little posies to give as a gift. Very old-fashioned, but classic.