What do you think about Ladybells (Adenophora confusa)?
I was a bit hesitant about adding this to my garden some years back, but thought it might be worth the risk. I've always loved coming across these around abandoned buildings or in waste areas along alleys. They just looked like such survivors and are so pretty in a casual sort of way.
Anyway, I've really come to like it and it hasn't turned into the thug I thought it might. After about 4 years in the garden, the clump has gotten larger, but it hasn't spread wildly throughout. Nothing seems to munch it either.