re: Long weekend 3-4 days anniversary trip: Boston or D.C.? In August

Depends on what you like to do, but there's plenty of great things to do/see. Incredible beer scene, if you're into that. The Saints are playing that first preseason game up here, I can't wait to go to that. There's obviously tons of historic stuff to explore (freedom trail, etc.), and a great food scene.

Boston is also very close to tons of other areas that are easily accessible: Portland Maine and Newport Rhode Island immediately come to mind.

There are also coastal towns that are easy to access by train where you can get lobster right out the water and go on whale watching tours.