I went on a garden tour last year and they had the most wonderful hollyhocks growing. I have a picture at home I can try to post. They were quite tall and impressive. I don't know much about how easy they are too grow. Are Hollyhocks biennials?

Rudbeckia Herbstonne has a rep for reaching 6-7 feet. I have some seed if you want. (As well as the castor bean)

How about amaranthe? Makes for neat cut flowers and the birds may eat it in winter, although mine here don't really go for it all that much.

Those are all great ideas! Thank you very much.