In LGA, you can walk right out baggage claim and catch the q33 to roosevelt ave train and take that into grand central station, there are a gazillon things to do in NYC.
In DCA you have the metro right across the street from national and Air and Space is awesome to visit, along with numerous other monuments and museums.
Just go on the internet and look up the cities, they all have visitor bureaus.
Phl has the quincy market, and BOS is full of history also.