Another one of my late summer staples is Verbena bonariensis. It is a perfect plant for a small garden as it takes up very little room in the border while creating a dreamy purple cloud of flowers when planted en masse.
It also self-seeds very enthusiastically, which is very obliging. The only problem is, it never lasts for long: one plant will see you through only a few years, so make sure you keep plenty of seedlings on standby just in case.