They had a good segment on this issue on Iowa NPR this morning. They said the last area of Iowa that remains pretty much unspoiled prairie is the Loess Hills out west. So this is basically what it would have looked like in pioneer times, pre-farming:
There is a trail that runs right at the top of the ridge going away from the viewing platform. I know prairie wildflowers pretty well, but we walked it last year amd my mind was blown; I had never before seen half the different flowers blooming all along that ridge. In the picture it just looks like grass, but it's rich with prairie plants. Plus the views are great.