As a Michigander, I highly recommend Greenfield Village and the Henry Ford Museum. It's a two hour drive from Sandusky (Cedar Point). You could do Cleveland first, then Cedar Point, then Greenfield Village. You need at least a full whole day to do the museum and village justice. It'd be better to split it into two days. If you wanted to go to Niagara Falls from there, it'll take you about five hours, through Detroit and into Ontario. For that route you'd all have to have passports to get through customs. But then you'd have a really long haul (16 hours) to get back home again. Or you could skip Niagara Falls this trip and head back west across Michigan and stop in Chicago on the way home. It's one of our favorite cities to visit from Detroit, only 6 hours away.

I love Niagara and Gettysburg, but in just a week I don't think you could add them without turning the trip into something really rushed. My suggestion would be to save them for another time so that you can have two days at Cedar Point and take your time at the Halls of Fame. If you really want to add one more, then Niagara is just a few more hours past Cleveland, as opposed to Gettysburg, which would eat up another day just getting there (plus it really deserves more than a few hours).

My 2¢:
Skip Gettysburg: the (excellent) time-lapse Civil War graphic at the Lincoln Museum would give the kids a better sense of the war's ebbs and flows than would Gettysburg.

Depending on your goals, I'd suggest Chicago's Museum of Science and Industry, and maybe the Field Museum of Natural History. Shedd Aquarium's nice, too, but there are limits.

Since you'll be so close I would include an excursion to Kelley's Island - http://www.kelleysisland.com/ Both Gettysburg & Niagara Falls out way out of the way, if you want 'outdoorsy' try here - http://www.hockinghills.com/

Thanks for the replies and suggestions! I love Chicago too. We've already taken the kids there twice, and that's the only reason I didn't include it in our plans again. But it's always worthy of consideration...

Since you are driving in that direction stop in Shipshewana Indiana. It is a neat Amish community. We like to eat at the Blue Gate restaurant and also shop at Yoders Store. They have a big flea market on Tues and Wed. There are many Amish made furniture stores in Shipshewana and we did buy 8 oak dining chairs.

Along Lake Michigan on the Michigan shores are the sand dune.
http://www.nps.gov/slbe/index.htm

I suggest taking a diversion...come north to Petoskey, traverse city, charlevoix. Beautiful this time of year. Great lighthouses, beaches, people.