Southwest flies to/from Hartford CT, Manchester NH, Baltimore MD, Chicago Midway, and Washington DC, usually for prices less than $100. We regularly fly Hartford/Baltimore for around $70, for example. Also they have no problem selling you a one-way ticket at a reasonable price.

Southwest does not participate in expedia, kayak, etc, so you won't find their prices listed on those websites. You have to visit the company website southwest.com

Southwest also flies into Boston (as well as Providence, RI).

The Washington, DC, airport Southwest flies into is Dulles (IAD), which is quite a ways out from city. If possible, you want to fly from Reagan National (DCA), right across the Potomac River from Washington.

The outer Cape, from Eastham to Provincetown,2+/- 1/2 hours from both Providence and Boston so if you use either of those airports and do not want to spend 5-7 hours in the car getting to Acadia, you should consider the cape. Although the Cape Cod National Seashore area is not as vast and rugged as Acadia, you will find many diverse trails, salt marsh and harbor area to explore. http://www.nps.gov/caco/planyourvisi...CACOmapweb.pdf

Kayaking is easy and convenient on the outer cape, the bike trails are well maintained and extensive enough to keep you busy and the light is a photographer's dream! I am sending along some links that might help you organize if you decide to choose Cape Cod. If you need lodging suggestions let me know.
http://www.capecodbiketrails.com/
http://www.nps.gov/caco/planyourvisi...bikerevise.pdf
http://www.nps.gov/caco/planyourvisit/things2do.htm
http://www.trails.com/activity.aspx?area=14651
http://www.massaudubon.org/Nature_Co...leet/index.php
http://www.cctrails.org/wellpond.htm
http://www.explorecapecod.com/
http://www.goose.com/
http://www.artsdunetours.com/
http://www.whalewatch.com/

Another vote for Cape Cod, especially the outer cape. Very close to Boston, beautiful. Many varied experiences, and in early September booth the bay and the lovely freshwater kettle ponds are swimmable, something that cannot be said for Maine. Truro, especially has gorgeous windswept dunes , Provincetown has a wonderful arts scene and shopping as well as excellent restaurants in every price range, whale watches, dune tours. kayaking is varied and excellent.

This place gets good reviews on Cape Cod:

http://www.fodors.com/world/north-am...ew-200775.html

Cape Cod is really nice in late summer. I have a couple of pictures, taken a little later, at www.flickr.com/photos/emalloy2009/sets in the Cape Cod set, if you care to look.