One absolute stop near Chicago is Calumet Fisheries.
Calumet Fisheries was named one of the five honorees for the 2010 James Beard Foundation Awards America’s Classics category.
I will also go out on a limb and toss in Joe's Gizzard City.
It's a hole in the wall but is a throw back. I love that the draft beer comes in Mason Jars, the Deep fried cheeseburger is not to bad, and I actually love the Gizzards. I think that qualifies as "Interesting". :)
Joe's Gizzard City
120 W Main St, Potterville, MI 48876
If you want to go the fast food route and are coming by way of Sarnia / I-69 there's always Halo Burger, a small Flint-based chain that has its devotees. I haven't been to one in ages but maybe someone else here can give a current assessment. There are at least 3 that aren't too far from I-69 exits: Court St (E of Flint), exit 139; Saginaw (downtown Flint), in a cool old building; and Linden Rd (W of Flint), exit 131.
Detroit's Mexicantown is very near the Ambassador Bridge. I don't know if you have good Mexican in TO, but there are some interesting options for both dining as well as groceries and bakeries in Detroit. My favorite restaurant is Los Galanes.
Middle Eastern in Dearborn, many choices. Also in Dearborn, not far from the freeway is Millers Hamburgers, which finds its way on many "best hamburger" lists, although you will get an argument from some Hounds :)
Of course Zingermans in A2 is quite famous.
When we drive to Chicago for some reason we seem to always stop for lunch at the Steak n Shake near Kalamazoo...not exactly chowish but they do have good shakes and a decent semi-fast food burger. EZ off, EZ on
We did a slightly different version of your trip last summer, driving from Toronto to Milwaukee over two days.
We managed to find both small mom-and-pop diners slightly off both 401 in Canada and I-96 in Michigan around Grand Rapids. Neither one was all that memorable (I don't remember the names), but they were tasty enough and not too terribly hard to find once we got off the 4-laners and put a little effort into looking around.
We spent the night in Dearborn, Michigan after crossing at Detroit/Windsor (which took us about a half hour to get through). Dinner was at Andiamo's, an Italian place in Dearborn. I know it isn't the most popular place on some other threads, but we enjoyed it especially attentive customer service.
In Muskegon, we took the Lake Express Ferry across Lake Michigan to the south side of Milwaukee. If your destination is the north side of Chicago (and you're not traveling during winter), I suggest taking it and making the easy drop south to Illinois instead of fighting that Chicago/Indiana traffic. Relaxing, peaceful (not cheap but worth it) ... and bringing this back to food, a serviceable on-ship snack bar with turkey panini's. It may not be gourmet, but that's surely interesting! :-)
There are two main routes between Toronto and Chicago. The quickest is to cross into the States at Sarnia/Port Huron and continue on I-69 in the States. Not that much of this route is along I-94 (or "the" I-94, LOL!); by the time you meet up with I-94 near Battle Creek you may as well keep going on to Chicago, because you're only a couple of hours or so away. This route doesn't pass through any big cities. It goes through Lansing, for which there's a discussion at http://chowhound.chow.com/topics/511240
You can cross at Windsor/Detroit but most people avoid doing so, not only for general congestion in the Detroit area, but also because of long backups at the border crossing. There are numerous topics here on Chowhound about food in the Detroit/Ann Arbor area.
Google suggests (obviously not always the best route) going through sarnia but taking (no the ;) )I-94 straight to Chicago, which brings me past Detroit and Ann Arbor. I am not in a huge rush and I can take one route back and the other there but which one would have greater potential for good roadside chow? Also, I'm going in three weeks so it'll be the dead of winter.