For photographers, check out The Photographer's Guide to the Maine Coast and let it be your guide - just don't forget there's more to Maine than just the coast.
Portland
Although there are many notable towns in Maine worth visiting, stop in Portland and go to the Old Port. Find a Chow-dah house on the dock and get some authentic Clam Chowder or Lobster. You'll never want seafood from Red Lobster again.
Lighthouses
If you are a lighthouse buff, moving from the Great Lakes to Maine is like winning the lottery. The architectual styles are much more diverse here, but like everything else - getting there is half the fun. Portland Head Light, The Nubble (Cape Neddick), and Pemaquid Point are some of the most photographed.
Acadia National Park
No trip to Maine is complete without stopping in Bar Harbor and Acadia National Park. From the Otter Cliffs, Thunder Hole, and Cadillac Mountain, to the quaint fishing villages and miles of hiking trails you'll find something to enjoy here. If you are looking for something a little easier on the wallet, try staying at one of the many bed and breakfasts during the off-season. The only downfall is many shops and restaurants close when the tourists leave.
Crawford Notch
This area was 15 minutes from my back door when I lived in Bethel Maine. Step Falls, Screw Auger Falls, and Table Rock are some points of interest. And don't forget the Newry Bridge - there's nothing like dangling your feet in the cool mountain water on a hot summer day.
Moosehead Lake Region
Mt. Katahdin, Baxter State Park, and plenty of Fall color - not to mention the moose (which were a little more elusive that I had hoped for). Unfortunately I didn't get to spend much time here.
Rangeley Area
Take the road from Rumford to Rangeley and you'll hit a number of waterfalls (Coos Canyon, Angel Falls), you can pan for gold, take a hike on the Appalachian trail, do some fishing, and watch Moose - all within an hour's ride. One of my personal favorites is to take in a sunset from the Height of the Land overlook. Spectacular. It's worth putting the camera down and just enjoying the show.