I'll second BostonDiner on that one. August is pretty brutal in NE (although not nearly as bad as the South can get). If you stay around the coast or even the mountains, you should be fine.
For the coast, York Beach in Maine is very nice, then you could travel down to Portsmouth, NH, down to Boston. Also, Providence and Newport, Rhode Island are great places to go (especially Newport...nice coastal town).
As for the mountains, Burlington, Vermont (mainly a college town, but it's a nice visit)...then head down to the Berkshires in Western Massachusetts (never been, but a friend from Norwich, Norfolk visited and loved it), perhaps from there down to Connecticut and over to Rhode Island.
There's a lot you can see in the New England region within two weeks...heck, it only takes 4.5 hours to get from Boston to Montreal, Canada (just to give you some indication)...and Boston to York Beach in Maine is only about 2 hours (give or take).