That's an ambitious trip for 1 week from Toronto. I can't suggest anything for NYC but they may have tour buses you can get a tour for a day or from your hotel that would hit the high spots, same in D.C. Don't forget going back to Toronto from D.C. is 800 miles. I'd suggest flying if you can swing that. Especially with a baby. It would give you more time to see the sites other than one day in each place. The must see places in D.C. are the Washington monument(good view on a clear day), the Capitol building, the Smithsonian museum of natural history(they have a stuffed Dodo bird and the Hope diamond)and Arlington Cemetery for the changing of the guard at the tomb of the Unknown Soldiers (also good view of Washington from the Cemetery).