What perennials can I use to plant in my window flower boxes?

I just put up 3 large window boxes, I know late in the season. I am planning for next spring and summer. These window boxes are 6 foot and 8 foot in size, they are 10 inches deep so I can plant perennials. I want to be able to keep my cost down each year so I was looking for ideas of plants that I can use year after year. I live in Ohio (zone 5). I want some plants to hang over the edges and ones that won't become too big and over power the whole box. Ideas? Thanks so much!
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