Hi Jim,
Maine Narrow Gauge (MNG), Boothbay Railway Village (BRV), and Seashore Trolley Museum are all open Summer weekdays. MNG tends to run steam on Saturdays. Both could easily be covered in one day - unless you really like streetcars/traction. However, much of Seashore's collection is unrestored and not generally accessible.
The WW&F is Saturday/Sunday (with Saturday usually the better day to visit, as we have more crew available to show folks around; Steam both days.) If you show up to the WW&F on a weekday, you are most welcome, but will not likely find anything running.
You could do the WW&F and BRV in one day, but you would likely feel rushed at one place or the other. BRV has a short train ride, but has lots of old buildings, a large auto display, model railroad, exhibits etc. BRV's weekday steam schedule, if that matters to you, should probably be confirmed before visiting.
A good lighthouse to visit (in the basic vicinity of the WW&F and BRV) is Pemaquid Point. If there is a "typical Maine" lighthouse, this is it.
Keith suggested the Sandy River (http://www.srrl-rr.org/) their schedule can be a bit more sporadic. They are also a bit further off the beaten path. They have a gas-powered "steam outline" which is OK. Their collection of rolling stock, buildings, and roundhouse/railyard make up for the lack of steam. You could also explore when they aren't open (just hike in on the trail [less than a mile] that follows the ROW), but everything will likely be locked up.
Acadia is very nice, but its another 2-3 hour drive north from the WW&F/Boothbay region. Portland is an hour south from the WW&F.
If you do head to Acadia, another good take is the Cole Land Transportation Museum in Bangor. That could be fit in during a trip to/from the Acadia area. The Penobscot Narrows Bridge & Observatory & Fort Knox is also a good stopping point going to/from Acadia (but via coastal Route 1, instead of 95 via Bangor.) I also second Keith's recommendation of Owl's Head transportation museum - you can grab a shot of the still-active ex-Maine Central roundhouse in Rockland.
There is a small NG steamer on display in Rockport, just up the coast from Rockland. Not really anything special (and not even a Maine locomotive), but worth a mention if you "collect" seeing such displays.
Let us know when you are coming, and we'll be sure to welcome you.
Ed Lecuyer
Trainmaster, WW&F Railway Museum
Help the WW&F Build Locomotive #11: [
build11.wwfry.org]