Here are a few small-statured shrubs (no taller than three feet) that have a great impact without trouble or disease or pruning: shrub roses like 'The Fairy', dwarf mock oranges such as Philadelphus 'Snowdwarf' or 'Miniature Snowflake', Fothergilla gardenii, and Oregon Grape Holly (Mahonia aquafolium). Shrub roses like 'The Fairy' bloom nonstop. Fothergilla has spectacular red, orange, and golden fall color. Oregon Grape Holly has dark glossy green foliage and berries that the birds will enjoy. You might also think about Spireas (Spiraea species) and Crimson Pygmy Barberry (Berberis thunbergii 'Crimson Pygmy'), but these are very common and sometimes overused, so they can sometimes look tired or ordinary in a landscape. However, if they're planted in an attractive combination with other things, they can look spectacular. It would be a good idea to include a perennial or two or three, to add diversity, interest, and color to the plantings. Siberian Iris will form wide, tidy clumps crowded with purple or pale yellow blooms. As they grow, the center of the clump will get bare, so they need to be divided once every 3-5 years. Reblooming daylillies (Hemerocallis species) are extremely popular and sometimes overused, but I love the bright clean yellow of 'Happy Returns', and I love how carefree they are. An ornamental grass will add seasonal interest; most come into their prime in late summer and fall when blooming plants can start to look spent. Because you want a shorter-statured plant, go for Fountain Grass (Pennisetum alopecuroides) or Dwarf Fountain Grass (P. alopecuroides 'Hameln'.