Website for you:
http://www.mainelighthousetrail.com/index.html
The good thing is you can see a whole bunch of them in Casco Bay (Portland) and then within an hour or so south/north of Portland.
Portland Headlight is the "classic" one to see- commissioned by George Washington. Up the road on either side you have Two Lights State Park, Portland Breakwater, a tiny little cute "Bug Light" and out in the harbor you can see Ram Island and (on a good day or out in a boat) Halfway Rock. (That one is WAY out, you need binoculars or a boat.)
In Boothbay Harbor, Burnt Island Light is neat. Not only can you go in it (most lighthouses you can't go in- see below), but it's a living history setting. Actors portray the lighthouse keeper and his family living there in 1950 or so. You do have to take a short boat ride there.
Up past that is Pemaquid Point (famous for being on our state quarter).
As said, you can't go into most of the lighthouses. A couple are on private property so you can't even go up next to them or anything. Many of them open to the public though on Maine Light House Day (2nd annual was this year, I suspect they will continue it). That is in mid Sept. Portland Breakwater used to open up on summer holidays- Memorial Day, July 4th and Labor Day. Not sure if they still do or not. That one is tricky, children under 10 not allowed and you do need to be able to climb up a short but steep ladder to get into it.