If you go to Michigan, there's no reason why you have to spend all or even most of your vacation time with the relatives. Make the visit a small part of your trip to break things up. I'd suggest hitting an amusement park like Cedar Point on the way up or back. And extend your trip to a drive all the way around Lake Superior, or at least go north of Duluth along the "north shore" or Superior; this area is chock full of waterfalls, trails, and lake views (Palisade Head is spectacular).
Leave your vacation as flexible and open as possible. If you all get the itch to head out farther and see mountains(say, to Colorado or Wyoming), or you feel like pointing yourselves back southward and maybe see the museums in Chicago, don't hesitate to pull up stakes and do what you feel like doing. Have a range of ideas and places as possibilities, then be spontaneous.
