Will you be coming on the Pennsylvania Turnpike via Pittsburgh? If you do, you've got about a ten hour drive into Philadelphia to begin with (and the same back, I'm assuming, so that really takes two of your days.) You could stop in Pittsburgh, and then again in Harrisburg or Lancaster, to break up the drive. Coming via Niagara seems as if it may be longer? I've never driven it, so can't give first-hand info I'm afraid, but at least this will raise your question up again.

What are you interests and when will you be going? If you're planning to go in October or November, that can help eliminate suggestions to stop in Cleveland or Pittsburgh for a baseball game or for great beaches.

In Cleveland, there's the Rock & Roll Hall of Fame. The city also has a good art museum. Part of the museum was closed for expansion, but may have reopened, check www.clemusart.com. It's located on University Circle with the botanical gardens, museum of natural history and Case Western Reserve University.

There's the Football Hall of Fame in Canton, Ohio and Pittsburgh has the Andy Warhol Museum. You could go to State College, PA and catch a Penn State football game or see the Capitol in Harrisburgh.

I agree that Niagara Falls is pretty far off the path between Detroit and Philly. Also remember that if you cross the border, you'll need passports to return to the US.

I am making this same road trip from detroit to philly. on the way there, we are stopping in Pittsburt at mt washington to ride duquesine incline for the view. I need a stop on the way home to break up the drive. any suggestions?

Depending on which part of Philly you are headed for take I-80 (freeway) east from Youngstown. You can take US 322 from State College through Harrisburg or continue east on I-80 and take the Northeast Extension of the PA Turnpike. Either way it is between 588 and 611 miles. Plan on 10 hours of driving time plus any stops.
Niagara Falls is not on the way.

I am planning on 10 hrs and know that Niagara falls is not on the way, but is there something historical, fun , or interesting to stop 4 or 5 hours into the trip?

Without adding to much to your time, you could stop in Altoona, PA, for the Railroad Museum/Horseshoe Curve and some other sights/sites: http://travel.webshots.com/album/582683132ahdfpw

On the way out of Altoona toward Tyrone there is a nice little family amusement park, DelGrosso's: www.delgrossos.com

Or, if you are interested in history, about the same amount of time will take you to Johnstown and its Flood Museum and such.

Maybe 6 hours from Detroit, you could visit Penns Cave. America's only water cavern. I have been through this cave more than once. http://www.pennscave.com/
You could go east on Rt. 192 after you leave the cave and see a few miles of the Bald Eagle State Forest where I once worked.

thanks Amy & Tom. I'll look into those.

As you travel east of Pittsburgh, take the time to visit Shanksville and the Flight 93 National Memorial
http://www.nps.gov/flni/index.htm