We're traveling to northern Indiana the weekend of the 17th, visiting my sister in Chicago the 18th and 19th, and then we don't have to be back in NE PA until the 23rd.
 
OK, then. We have a couple days to get around the Great Lakes area. Not thinking just eats, although that would be part of it, but we have a couple days to just go SEE something. DO something. But WHAT?

I don't know what you have already seen, but here is a selection of some well-known and not-so-well-known places:
 
CHICAGO, IL, area:
Museum of Science & Industry: http://www.msichicago.org/
Art Institute: http://www.artic.edu/aic/
Field Museum: http://fieldmuseum.org/
Shedd Aquarium: http://www.sheddaquarium.org/ 
 
SANDUSKY, OH, area:

Cedar Point Amusement Park: http://www.cedarpoint.com/

Merry-Go-Round Museum: http://www.merrygoroundmuseum.org/

Ghostly Manor: http://www.ghostlymanor.com/

Maritime Museum of Sandusky: http://www.sanduskymaritime.org/
African Safari Wildlife Park (Port Clinton): http://www.africansafariwildlifepark.com/
 
CLEVELAND, OH, area:
Rock and Roll Hall of Fame & Museum: http://rockhall.com/
Great Lakes Science Center: http://www.glsc.org/
Cleveland Museum of Art: http://www.clevelandart.org/

I don't know what your interests are, so I will just toss out a few ideas that are not too far from the Indiana and Ohio turnpikes.
 
Shipshewana, Indiana, is a village in northeastern Indiana's Amish country, and can be worth a stop with its stores & shops geared to both the Amish and the tourists.  They also have a very large flea market there every Tuesday and Wednesday.
 
The greatest concentration of Amish is in Holmes and adjoining counties in northeast Ohio, about 100 miles from Pittsburgh. Next in size is a group of Amish people in Elkhart and surrounding counties in northeastern Indiana. Then comes the Amish settlement in Lancaster County, Pennsylvania.
 
There are a good many wineries in southwestern Michigan that have tours and tastings, roughly centered around the town of Paw Paw.
 
Holland, Saugatuck, and South Haven are nice beachfront towns to visit on Lake Michigan.
 
Michigan has a large number of lighthouses (more than any other state) -- the state consists of two peninsulas connected by one bridge over the Straits of Mackinac.  Michigan's peninsulas are bordered by Lake Superior, Lake Michigan, Lake Huron, Lake Erie, Lake St Clair, the St Clair River, and the St Marys River, and all, including the rivers, at one time had lighthouses.  Many are still there, although they may not be use any more.  Some people just visit these lighthouses.
 
Restaurants
Blue Gate Restaurant, Shipshewana, Indiana (Amish region)
Schmucker's Restaurant, Toledo, Ohio (reviewed on this site)
Tony Packo's, Toledo, Ohio (reviewed on this site)
Village Inn, Middlebury, Indiana (reviewed on this site)
Clay's Family Restaurant, Fremont, Indiana (good roadfood)
 
Auto Museums
Auburn-Cord-Duesenburg Museum, Auburn, Indiana
Gilmore Car Museum, Hickory Corners, Michigan (Kalamazoo)
Hostetler's Hudson Auto Museum, Shipshewana, Indiana
National Automotive and Truck Museum, Auburn, Indiana
S. Ray Miller Antique Auto Museum, Elkhart, Indiana
Studebaker National Museum, South Bend, Indiana
 
Other Museums
Air Zoo, Kalamazoo, Michigan
Henry Ford Museum, Dearborn, Michigan (includes Greenfield Village)
Sauder Village, Archbold, Ohio

Mosca, if you haven't been there before, the College Football Hall of Fame in South Bend is worth seeing.  And I am a fan of the burgers at Redamak's in New Buffalo, Michigan, 

buffetbuster


Mosca, if you haven't been there before, the in South Bend is worth seeing.  And I am a fan of the burgers at in New Buffalo, Michigan, 

Agreed on both accounts. You can do both the College Football Hall of Fame and the aforementioned Studebaker Museum easily on the same day.

OK. Marblehead Lighthouse at Marblehead, Ohio on Lake Erie, with a stop in Port Clinton at the Jolly Roger for walleye and/or perch sandwiches (the onion rings are great). Canton, Ohio -- the Pro Football Hall of Fame.

Mosca, you could visit the Nite Owl Drive In in Milwaukee.  I'm sure it'd be open, what with you being from Pennsylvania and all.
 
Since your daughter goes to Notre Dame perhaps Holy Hill in Hubertus, WI might be of interest to you.
 
Brad

Let me add another hearty thumbs up to Redamak's in NewBuffalo. While traveling across Northern Indiana I would suggest that you stop in the Rise and Roll Bakery just east of Middlebury, In on US 20.  IMHO the finest donut on the planet can be had there.  A cinnamon caramel donut that is absolute nirvana.  They also have an incredible breakfast of sausage bread with sausage gravy that is superb (and onyl $2.99)!  Just down the street to the west is Das Dutchmen Essenhaus which serves wonderful home style meals and has several shops, a hotel and other things to do.
 
Further west in the town of Elkhart is the Midwest Museum of American Art that is chock full of paitnings pictures, sculptures, etc and a real gem.  There is also a good railroad museum if that is your thing a couple blocks south. 
 
If Amish Country interests you than stop in at the Elkhart County Visitors Bureau just off of the toll road (exit 92 across through the red light and on your right) and you can pick up a CD that will guide through the area. 
 
If you enjoy quirky art you can also find a map to see the 29 decorated Elk sculptures located throughout Elkhart County right now.
 
In Chicago I would highly recommend Hot Doug's for some of the finest hot dog/sausages you will find anywhere and not just hot dogs but gourmet sausages and toppings that are inventive and delicious (reviewed on this site).
 
If you enjoy driving around the lake definitely hit the small towns and villages that go along the eastern shore of Lake Michigan.  Lots of fun shops, restaurants, etc.
 
Have a great trip!

I was thinking some more about things to do.  If you have never been to Grand Rapids, Mi it has got a lot of really great stuff going on there.
 
In the downtown area there is the GRAM (Grand Rapids Art Museum) which is a truly world class art museum with some amazing pieces.  There are 4 or 5 other museums within walking distance.  I don't know your budget but I would recommend staying at the Amway Grand Plaza (stay on the historic side for a truly beautiful property) and it is close to great little ethic restaurants, clubs and the museums.  On the northern side of GR there is the Frederic Meijer Sculpture Gardens.  Home of the "American Horse"....this truly has to be seen to be appreciated.  It really is a vibrant city and still has sort of a small town feel and the pricwes are definitely better than Chicago.