My first thought is herbs. I grow loads of herbs in pots on the patio and they look (as well as smell and taste, of course!) fabulous. I have various types of mint (although watch out these will take over), various sages, fennel (which is wonderful as it grows tall fast, but is light and airer so never gets in the way), rosemary. I think hostas, well watered and not in too much sun, would work well too. I really don't like their flowers, so for the few weeks I had some (before the slugs ate them) I pinched the flowers off.
For something bigger you could try New Zealand flax. Again this has lots of different shades of green to choose from.
