There's basically two obvious choices for getting from the Chicago area to Niagara Falls: either you stay to the south of Lake Erie (which pretty much means taking I-90), or you go north of the lake through Ontario.
I myself would probably prefer the northern route. You do, of course, need to take care of the necessary border crossing things like having passports and not having banned items. I-90 works, and is quick, but it gets kind of boring and toll-y.
The Henry Ford museum would be more or less on the way for the northern route (if you aim for Detroit rather than Sarnia), and is very much worth a couple of days to visit.
I would definitely continue East before turning around. Upstate New York has some beautiful areas (the Finger Lakes and the Adirondacks particularly, in my estimation). Letchworth SP and Watkin's Glen SP (in the Finger Lakes) are mighty fine, and are pretty nearly on the way to Gettysburg etc.
The Corning Museum of Glass is much more interesting than its name might suggest.
