Sunflowers might be an option. Here I would expect a sunflower crop to make something decent being planted 8 days later a soybean crop. But being how far north you are and lack of having hot weather this might not be true for you. With the right amount of moisture sunflowers will grow fast on days that are 95 to 100 degrees. With a full profile of moisture on good Iowa farmland you should be able to grow a decent crop of sunflowers without any rain. Price on sunflowers at harvest last fall was horrible.