Gauras are lovely, and if their conditions are good they can get very tall. They flower quite late, so extend the season. It looks like lots of butterflies flitting about.
I use them, also alliums (there are some tall spindly varieties), many of the varieties suggested, but also white lychnis - lychnis coronaria alba. With silvery foliage as well, it has a long season of interest and gets to a nice height. I have a whole bank of it in front of a pond, I like to see through it to the plants on the other side of the pond.