Bar Harbor and Acadia National park are just awesome! In Bar Harbor make sure you check out Ben & Bill's Chocolate Emporium, absolutely the best ice cream I've had anywhere! The whole town is great. You can rent bikes or kayaks or whatever.
the beaces are fantastic, esp in the Southern part of the state. Long Sands in York has a massive beach when the tide is out. old Orchard and Ogunquit are great beaches too. Ogounquit is known for it's sizable LGBT population, so if that's a problem with your travel partner, just be aware.
Moosehead Lake in the middle of the state is beautiful. You can usually find cabins and campsites pretty cheaply. You will more than likely see a moose in the area, it's just loaded with them!
Portland is a great city, like a small Boston or a larger Portsmouth NH. Something for everyone, great food, beautiful harbor area.
I live in NH, but go to Maine often (I hate shopping and so does my wife, so I can't vouch for any of the shopping stuff). To me, Maine is def an "outdoors" state. I used to love the lobster up there, but no more!
Check out http://www.visitmaine.com/