Never been to Dc but Philly is an amazing place to shop for clothes and shoes as its TAX FREE! so if a customer is doing a Twin Centre holiday or combining it with somewhere else in the states make sure they get there shopping here as it'll be much cheaper :)
The Liberty Bell and Independence halls where the declaration of independence was signed are must do's here...
For Philadeplpihia, the Reading Terminal Market is a must. The Big Bus Tour is a great way to see the cities main sights too, really enjoyed that. Fab restaurant, which used to be a bank and vault, is Del Friscos - amazing food and great cocktails!
For D.C, apart from all the normal must see sights, its worth a trip across to Arlington, Virginia to the National Cemetery. They do a hop on/off type tour once you're there which takes you around the cemetery and tells you about all the main graves etc, Kennedy & Jackie Onassis eternal flame, the Tomb of the Unknown Soldier and the hourly changing of the guard there. Very moving, but well worth going. A great restaurant in DC is the Founding Farmers in Foggy Bottom - great for breakfast, brunch or dinner, and advise booking in advance as its very popular!