You've left it rather late for plants to make much of a showing this summer. I'd go to your local garden centre and see what they have left. Try to go for masses of the same thing, rather than spotty bits and pieces. I'd avoid Busy Lizzies as they seem to be having a bad year this year. Lots of people are finding they're dying off early.

How about some heucheras - the coloured foliage will last for more than just the summer. If you bought one or two plants of stipa tenuissima as well (a small feathery grass), this will self seed for future years. The combination of different textures will be understated but very effective