Another vote for House on the Rock. Simply amazing. The area surrounding the House is all dairy area - very pretty area for just driving through.
While in Chicago, don't forget to visit Millennium Park - downtown Chicago. Some of the features of the park are the incredible Band Stand (an architectural delight - even if there is no music playing at the time - it's wonderful to look at!), the "Bean" - a stainless steel sculpture that mirrors the city skyline, and the Millennium Fountains - wear shorts and sandals because wading is not only permitted but encouraged!!