Which way does your garden face, and how much sunlight does it get?
If it gets a half decent amount of sun, how about:
Lilies
Delphiniums
Dwarf Lupins.
They're all hardy perennials, i.e. will come back year after year regardless of frosts etc. The delphiniums will need some protection from slugs, but other than that they're relatively hard to kill once established.
They are however, quite big. If you want something that looks like this:
www.growquest.com/container%20gardening/contai35.jpg
Then what you probably want are annuals. You can buy these from any garden centre or B&Q/Wickes for a couple of quid, and they will give you flowers all summer long. Good ones to go for are pansies, osteospermum, lobelia, geraniums etc.
Great thing about annuals, is you only need to keep them alive for 6 months, then they're allowed to die, and you can either replant the containers with spring bulbs (tulips, daffodils etc) or just wait for summer to come along again.
Also if you've got a nice wall for a trellis, try something like a Clematis Montana that will colonise it very quickly.
Small(ish) perennials that do quite well in pots are roses, coreopsis and scabiosa.