If your DS loves flags, the Ben Franklin Parkway in Philly is perfect. It's a grand thoroughfare modeled on the Champs Elysees and lined with flags from every country in the world, with name plaques identifying them all. Right on the way to either the Franklin Institute or the Academy of Natural Sciences, both of which are fun (and have reciprocity with lots of other museums if you have a membership where you live).

Also on the flags, though off the beaten path a little, South Broad Street (south of Washington Ave I think?) is lined with all the state flags. And actually that's on the way to the ballpark if you take the bus instead of the subway, might be fun to see!