Since you're going to be spending a a day or two in each of Philadelphia, Washington, and Toronto, but don't like museums, here are a few great places to just walk around in those cities and get a feel for them.
Philadelphia: Independence Mall of course, but that's fairly museum-like in some ways; Elfreth's Alley including the nearby Betsy Ross House for a taste of colonial Philly; and the Italian Market along S. 9th Street for a taste of tastes.
Washington: The National Mall is lined with world-class museums, mostly various divisions of the Smithsonian Institute, but it's literally THE place to walk and see most of what you think of as Washington including the Capitol, the White House, the Washington Monument and the Lincoln Memorial, as well as 'smaller' memorials to Vietnam, Korea, and World War II among many, many others. I'd also suggest that you take a day and get down along the Potomac River where you'll find not only great walking trails along the river but also the Roosevelt and Jefferson Memorials, and the Tidal Basin.
Toronto: I'm not nearly as familiar with this great city, but Yonge Street is the heart of the metropolis and I have certainly enjoyed their zoo. I also found an outfit that offers free walking tours where you might get more ideas.
As I noted earlier, I probably wouldn't use Detroit as either a way point or overnight stop, but rather return to the US via Niagara Falls and follow the southern shore of Lake Erie to Sandusky, using NY-5/PA-5/OH-5 as much as possible, and I-90 when I needed to make time, while visiting a few lakeside parks such as Evangola, Presque Isle, and perhaps the Rockefeller Park Greenhouse. I will also suggest one, and only one, museum that your teenagers might like, the Cleveland Museum of Natural History.
And finally, while you're in the Sandusky area for Cedar Point Amusement Park, consider taking a day out on the water to one or more of the Bass Islands.