My first thought was Washington DC - we did this last summer and added a few days in Philadelphia - we were visiting colleges. Our group was myself, DH, DS17, DS13 and DS11. Yes, it was HOT - but we managed very well and everything is air conditioned. Watch "National Treasure" before you go - DS11 begged me to go to the National Archives!!! The boys still talk about this trip - the monuments, the Spy Museum, Ford's Theater (where Lincoln was shot), US Capitol building, Library of Congress, Holocaust Museum, Smithsonians, etc. So much to do and see - we've already talked about what we want to do on our next trip there! Oh yes, DS13's choice was Washington National Cathedral - amazing!! I believe there's lots of shopping too, but not for my group of guys!
Colonial Williamsburg/Busch Gardens/Water Country is also a great trip!