Posted: 31/05/2015 at 16:57
Hi angie - what sort of size is the window box and what aspect? That will help with suggestions, although I'm guessing it's reasonably sunny if you normally use annuals etc.
You could go for a more structured look using box balls and ivy - planting crocus in between them in autumn gives you an all year round effect. Some of the very small Hebes would give the same look as the box but with less effort. If you like herbs you could put a selection in there for a more informal look.